Want a brief rundown on modding Arzette? You’ve come to the right place! However, a note before continuing:
All modding/game modification is done at your own risk. Limited Run Games and Seedy Eye Software are not responsible for any damages or errors caused by the usage or installation of mods or other user-created changes to the game. No support, technical or otherwise, will be provided for users who have modified their game.

With that out of the way, there’s a few different ways to mod Arzette. This is a very simple, high level guide; I’m sure better, more in-depth guides will be written by the community.

These are pretty straightforward. The game’s audio is stored in the /audio subfolder in the install directory, and the game’s videos are stored in the /video subfolder in the install directory.

The game will attempt to load music files if they are the same filename and in the same format (OGG vorbis).

The game will attempt to load video files if they are the same filename and in the same format (.mp4). The chroma key/blue screen shader will target a bluescreen of RGB value 0, 71, 187. Subtitle timing is controlled by in-game scripts (see point 3 for more information).

Adding new translations to Arzette is easy as well. The game’s localization files are kept in the locale.csv and captions.csv files in the install directory. You can access the localization Google spreadsheet here[docs.google.com]. This is the sheet used for development so it may or may not change as the localization is updated for accuracy. You cannot edit this Google sheet, so make a copy and save it to your own Google account. Once you have your own copy, you can edit the game’s text to your liking and replace languages as you see fit. Once you are finished, export the selected Google Sheet spreadsheet tab (either locale or captions) as a .csv file and overwrite the existing file(s) in the install directory.

Non-English languages use a smaller font, so keep that in mind (for instance, if you want to keep the smaller font, edit non-English columns). Note that you will have to replace the Vinque font file in the game’s data (see below) if the font doesn’t support the characters you have in mind for the language you’re translating the game into.

To modify the in-game data (sprites/textures, scripts, code, etc) you will need UndertaleModTool[github.com]. This data is stored in the data.win file in the game’s install directory.
Unfortunately, at the time of writing, UTMT does not fully support the compiled GameMaker runtime that Arzette uses. Once it does, however, you should be able to replace the font file(s) (Arzette uses Vinque in its various sizes and types; due to font licensing restrictions, I do not include a copy of the font file itself with the game) and replace/modify sprites/textures and sound effects. You may also be able to modify the game’s code as well (subtitle timing is present in the scr_sub_* files, for instance, timed by milliseconds) but it may be difficult to do with the way the game is built.

There are much better guides for modifying GameMaker game data than I could write out here; in a nutshell, you modify the data.win file to your heart’s content, and then generate an XDelta patch[www.romhacking.net] to be applied to that data.win file that can be freely shared. Please do not share the raw data.win file you modified. Make sure you indicate which version of the game you're modifying as well!

That just about covers it! Please mod to your heart’s content and have fun! You can share your mods anywhere; GameBanana[gamebanana.com], in the community hub here, through social channels, in the Seedy Eye Software Discord[discord.gg], and more! Thanks for playing and enjoying Arzette.