In early 2026, complaints arose from Latin American fans regarding the lack of official Spanish and Portuguese localizations of Undertale and Deltarune. This stemmed from earlier complaints over merchandise producer Fangamer's high shipping costs to Latin American countries, and a lack of South American tour dates for the "Undertale: The Determination Symphony" concert tour, both due to financial and logistical issues on part of Fangamer and the concert organizers.

In response, creator Toby Fox released a statement explaining that the reason why the games' only official translations were in English and Japanese was because he wanted to provide direct oversight to localizers in languages he was already fluent in. Since the games' scripts rely heavily on wordplay, he wanted the translations to adhere as closely as possible to his intended vision to reduce the risk of translation errors, adding that he was "not against official translations for other languages if we could make it work the same way..." Regarding Deltarune specifically, Fox expressed concerns that producing more translations would prolong the game's already drawn-out development, and endorsed unofficial fan translations as a way to allow fans to "interpret the game without the burden of being "officially" perfect". This statement drew mixed responses on social media, with some defending his perfectionist creative vision and efforts to manage the scope of development, while others felt that his standards were unnecessarily high and that the financial success of the games justified the demand for more translations.

From Chapter 2 onwards, the game's title screen includes a small subtitle below the main logo listing the current chapter. In addition to including graphics for Chapters 2 through 7 (lining up with the visible options on the chapter select screen), the game's data also includes Spoiler:graphics for Chapters 0, 1, 8, 9, and 10. While the Chapter 1 graphic is presumably for organizational purposes, the role of the remaining unused graphics is more ambiguous. As the game states that "THE HALFWAY MARK HAS BEEN ATTAINED" after completing Chapter 4, it is unclear whether the references to Chapters 0, 8, 9, and 10 were reflective of an earlier, longer outline for the game's story or if they were included as red herrings.

During the final battle of Chapter 3, Spoiler:the Roaring Knight is able to render Susie and Ralsei's HP to -999 when knocking them out, with the results represented by red text reading "SWOON". This appears to reference the Black Sword weapon in Final Fantasy IV, which has a chance of knocking out enemies in one hit. In the English localization of that game, the text "Swoon!" shows up when this occurs. The Knight further references this phenomenon with its own sword, the Black Knife (which also serves as its battle theme's namesake).

In a retrospective article detailing the making of Chapter 5, developer Toby Fox revealed that Seth was originally designed with lavender skin and white hair. The decision to revise this idea came about after his friends pointed out their resemblance to the character Amethyst from the 2013 American animated series Steven Universe. Concept art featured in the article showcases an early alternate design that features a uniformly white appearance barring a purple flower collar and a purple crescent moon decal on Seth's robe. A second design depicted in the same art features the same purple palette depicted in the final game, but retains the collar. While Seth's final design drops the decal and replaces their collar with a tie, the sprite based on the white design's pose features the tie curving into a crescent shape, alluding to this early idea.

Chapter 5 features many references and similarities to the Touhou Project series, of which developer Toby Fox and illustrator Temmie Chang are fans, through outfits, items, character designs, in-game text, and music. These include, but are not limited to, the following:

• The Red Ribbon and Netskie Hat headgears are references to Reimu Hakurei and Ran Yakumo.
• Aqua's character design, which Fox admitted was directly inspired by the Touhou series, bears similarities to characters such as Cirno, Sakuya Izayoi, and Koishi Komeiji.
• Flowery's line "Star sign: the Flower", presented like a dating simulator profile, is formatted similarly to Spell Cards' names, while bullet patterns throughout the chapter resemble Spell Card patterns.
• In the Ideal Diner, a Floradinn repeatedly orders eggs with increasingly hardening yolks, referencing difficulty options and stages in the Touhou games.
• The sound effect for collecting Flowery Dollar coins is a whispered voice resembling the graze sound effect from the licensed fangame Touhou Luna Nights.
• The Phanta item's description calls it a "grape-flavored phantasmagoria of a soda's dream", referencing Touhou Yumejikuu: Phantasmagoria of Dim. Dream. Its drink dispenser description, "Purple soda in hidden flavor", and ability to grant 16 TP allude to the 16th mainline game in the Touhou Project series, Touhou Tenkuushou: Hidden Star in Four Seasons.
• In Yellow's first punishment room, the party can encounter Shinobeetles trapped in quicksand asking "(Won't somebody please help me?)". This references a 2014 tweet by Fox about a presentation slide in a class lecture featuring a photoshop edit of Sakuya sinking in quicksand.
• The chapter's soundtrack quotes several Touhou themes, including "Theme of Eastern Story", "Lovely Mound of Cherry Blossoms ~ Japanese Flower", and "The Doll Maker of Bucharest". "Petal Dance," the battle theme for most of the colored flowers, prominently features Romantic Tp, a trumpet soundfont commonly used in the Touhou games and nicknamed the "ZUNpet" by fans. The "Shop 3" theme mimics the sound chip featured in the PC-9800 Touhou games.
• There are two references exclusive to the Japanese version of the game. Bullet patterns are referred to as "Danmaku", a term used for them throughout the Touhou series, while Flowery's "smile again" voice clip references a similar line from the Touhou fan video "My Name is Hong Meiling".

The BromideF and BromideR items reference the collectable bromides from the Lunar series of RPGs, which similarly feature high-quality illustrations of the games' cast members that pan up and down with warm background music. The BromideF illustration (depicting Flowery in a bubble bath) further parodies Borgan's Bromide and Lemina's Bromide 4 from Lunar 2: Eternal Blue Complete. All three illustrations feature the same poses, settings, and composition, but Flowery is fully clothed in BromideF.

In Chapter 3, if the player enters Mancountry, the hidden area containing that chapter's egg room, and looks through the binoculars on the western peninsula, they'll see a field of gray grass. Later, in the Weird Route version of Chapter 5, Spoiler:Kris receives a loaf of bread from Sans as a key item, with its Check description mentioning its tendency to leave crumbs everywhere. Both of these reference a glitch observed in the original 1989 Famicom release of EarthBound Beginnings, which exploits a quirk of the Bread Crumbs item (in which creating it for the first time in a playthrough with Pippi, returning her home, and then reclaiming her inventory resets its warp coordinates to [0, 0]) in order to transport the player to an out-of-bounds area consisting of monochrome graphics pulled from the last area they were in. Most users of this glitch trigger the bugged Bread Crumbs in Podunk, with the view from Mancountry's binoculars being identical to the consequent out-of-bounds area. These allusions to the glitch foreshadow Spoiler:the climax of Chapter 5's Weird Route, in which Noelle and the SOUL forcibly drag her and Kris through Hometown's lake, apparently going out of bounds as a result.

In Chapter 3, inputting exactly 1225 points into the giant gachapon machine in the B-Rank room unlocks a hidden door inside the front of the machine leading to gacharoom_unknown, a large maze of red carpet against a scrolling blue checkerboard background. The maze has no collision data and loops indefinitely, but one pathway eventually leads to a smaller gachapon machine. Interacting with this results in Kris acquiring a capsule containing a "small, dark triangle", hard like lacquer, that they then lose before becoming unable to find their own hand. Kris is then teleported back to the B-Rank room, and the giant gachapon machine is permanently removed. The background music for this area, labeled "findher" in the game's files, was previously featured in a hidden page in the Spamton Sweepstakes website, where it plays upon clicking an image of Dess' guitar.

When presenting Spoiler:Spamton NEO's Shadow Crystal to Seam at the end of Chapter 2, Seam states that Kris, Susie, and Ralsei will be unable to defeat their next opponent without the aid of the Shadow Mantle. In Chapter 3, the item appears as a hidden piece of armor obtained after Spoiler:completing the Sword Route, and, true to Seam's word, it significantly aids the player's chances of defeating Spoiler:the Roaring Knight by increasing the wearer's resistance to Dark and Star element attacks. Despite this, it is fully possible to defeat the boss without equipping the Shadow Mantle. Doing so and then obtaining the resultant Shadow Crystal results in unique dialogue from Seam when it is presented to them at the start of Chapter 4, in which they express their astonishment at the party's feat.

At the end of Spoiler:the Sword Route in Chapter 3, Spoiler: when Susie comes to greet Kris after the latter obtains the Shadow Mantle, the player is secretly given control of HERO_SWORD again. Consequently, it's possible to guide them back into the room and to Kris and Susie, which Kris notices. Attempting to attack Susie during this results in Kris pulling her away to protect her, whereupon she lets them taste her hair as compensation for her drinking their shampoo. Afterwards, Susie's dialogue proceeds as normal.

Two tracks in Deltarune prominently interpolate "Merry CD", a track that developer Toby Fox contributed to the Omori soundtrack. The first is "d", which plays during the cutscene that precedes Spoiler:the Weird Route version of the first Berdly fight in Chapter 2. The second is "Gingerbread House", the theme that plays inside the Holiday family's mansion in Chapter 4. The choice of reference alludes to how both sequences prominently focus on Noelle, matching the holiday theming of the Omori song.

Within the files for Chapter 5 is an early draft of Noelle's portraits for Spoiler:the drowning sequence at the climax of the Weird Route. While most of these portraits feature slightly more pixel padding than their equivalents in the final game, the last four are substantially different in design. Additionally, the last two depict Noelle Spoiler:continuing to fade into the black background rather than cutting to a frame of her shouting euphorically. Additionally, the final versions of these portraits feature an unused final frame depicting Spoiler:a close-up of Noelle's eyes, likely intended for her final line of dialogue (which, in the released game, instead duplicates the shouting frame from the previous line).

In a retrospective article detailing the making of Chapter 5, developer Toby Fox explained that the chapter's platforming mechanics were conceived shortly after Chapter 2's release in 2021, and were developed as a prototype before expanding in scope late into Chapter 4's development. According to Fox, a major issue with the mechanic's development was allowing it to mesh properly with the standard RPG gameplay of Deltarune, resulting in the formation of the ACT system and the addition of dialogue and RPG-style encounters in these sequences. The ACTs themselves originally encompassed a broad range of ideas before being whittled down to using Susie's Rude Buster and Ralsei's hanging ability. Fox also stated that the developers had to consider the possibility of Deltarune being many players' first experience with platforming gameplay, especially since Chapter 5, a relatively late point in the story, would be the first time it would appear in the game. Consequently, the level design was simplified to keep the "actual jumping challenges" from becoming too intense barring a few optional areas.

On July 11, 2026, both "Cutie Mew Mew Magic" and "Flower Man" from Chapter 5 became developer and composer Toby Fox's first two songs to reach the Billboard charts, used to catalog popular songs in the United States. Both songs appeared on the Hot Dance/Electronic Songs Chart, with the former debuting at No. 18 and the latter debuting at No. 13. While soundtrack albums for Deltarune's prior chapters previously appeared on the Billboard soundtrack charts, this marked the first time where individual songs from one of Toby Fox's games charted.

Within the data for Chapter 5 is an unused cutscene depicting Kris and Noelle riding the Ferris wheel at the festival after aborting the Weird Route in Chapter 4. Noelle recounts Kris' assurances that Chapter 2's events on the route were a dream before expressing how "amazing" it would've been if the power she demonstrated was actually real. When Noelle attempts to interrogate a reluctant Kris as to how they knew about the dream she had, she gets interrupted by Susie throwing dirt at the carriage window, which brings Noelle back to her senses. This cutscene is fully functional, but cannot be accessed without modding the game due to its associated requirements being impossible to meet in normal gameplay.

In a retrospective article detailing the making of Chapters 3 & 4, developer Toby Fox stated that the sequence where Spoiler:the SOUL travels alone throughout the Holiday Mansion went through several disparate revisions during development to "make you feel like it wasn't wasted." Fox stated that he initially conceived "a Rube Goldberg machine, involving many crazy parts" which the player would need to utilize in order to manipulate character actions. Subsequently, this was replaced with a sequence where the player utilizes a model train set spanning the whole of the house. When this didn't work, he conferred with personal friend and Homestuck author Andrew Hussie, who suggested a puzzle requiring the player to complete a bust parodying Macaulay Culkin's pose on the theatrical poster for the 1990 Christmas comedy film Home Alone.

Ultimately, Fox opted for the final game's approach, in which Spoiler:the SOUL simply navigates the mansion vents to spy on various characters, after realizing that "just focusing on the story and atmosphere is the best." Despite this, remnants of the early ideas remain in the released version of Chapter 4. The tail end of the sequence includes Spoiler:a scene in which the SOUL has to manipulate Noelle and Susie into capturing it and moving it into the living room and a scene requiring the SOUL to manipulate objects in Dess's closet while distracting Kris. Additionally, Chapter 4's data includes sprites and a functional test room for the toy train, while various sprites associated with the final sequence feature the phrase "homealone" in their filenames.

In Chapter 5, if the player loses to the first Floradinn encounter, Kris, Susie, and Ralsei will wake up in three literal flower beds in a unique room in the Garden. Subsequently, Flowery will appear and reveal that he rescued the party after finding them "unconscious, washed up on the beach," expressing concern for their safety and discouraging them from continuing their adventure. Exiting this room places the party in the area where the first RedRibbon is obtained.

Within the data for Chapter 3 are unused sprites depicting brown, black, and gray deer; the gray deer acts as a hurt sprite for the black one. The brown and black deer are early versions of Spoiler:the White Cloak and the movement-copying black monster found on the Sword Route respectively. The former NPC's internal filename, "deer", is a remnant of this initial idea. Of note is that the game uses a variable named "toriel" to toggle these characters' sprites between the deer and the designs seen in normal gameplay.

In console releases of Chapter 5, when Kris, Susie, and Ralsei ascend the beanstalks leading from the Gardens to the Cliffs, the surrounding border scrolls alongside the level. This includes both the large stalk in the platforming section and the second, smaller stalk during the RPG section.

During the fight against Orange and Green, Orange opens the pair's first turn by commanding Green to "rev up those fryers." This appears to reference a line from the 2000 SpongeBob SquarePants episode "Arrgh!", spoken by the recurring minor character Fred as he enters the Krusty Krab. At a later point in the fight, Orange compliments Green's cooking skills by stating that "these nuttrons will have you rustling in your jimmies." The corruption of "nutrients" as "nuttrons", coupled with the word "jimmies", references "Jimmy Neutron Happy Family Happy Hour", a surreal internet animation parodying the 2002 animated series The Adventures of Jimmy Neutron, Boy Genius where the title character is mispronounced as "Jimmy Nuttron". Meanwhile, the phrase "rustling in your jimmies" itself is a reference to the memetic phrase "that really rustled my jimmies".

The allusions to two different cartoons produced and aired by the TV channel Nickelodeon likely alludes to the network's use of orange and green in its visual branding.