Under the hood, it modifies the calendar textures to rearrange the day of week labels. It is set to do this "late", so it probably maybe possibly works with reskinned calendars. It should theoretically work with every language.
It also detects and patches Content Patcher to agree that Sunday is the first day of the week for the DayOfWeek token.
It may cause issues with mods that do their own independent day-of-week calculations for whatever reason. Mods that use the game's built-in day-of-week methods should receive the modified day.
Methods patched are:
Game1.shortDayNameFromDayOfSeason
Game1.shortDayDisplayNameFromDayOfSeason
WorldDate.DayOfWeek
ContentPatcher.Framework.TokenSaveReader.GetDayOfWeek
Technical
Requires SMAPI, uses Harmony.
Compatible with Mod Updater for automatic updates.
Code is at https://github.com/aedenthorn/StardewValleyMods.
If you want to complain or ask for help or help me test my mods, you can visit my Discord server.
A list of all my mods for Stardew Valley is available at https://www.nexusmods.com/stardewvalley/articles/895.