Просто вставте "ukrainian.json" у папку \SeamlessCoop\locale, та у файлі "ersc_settings" змініть це:
[LANGUAGE]

;Leave this blank unless you want to load a custom locale file. The mod will default to your game language.
mod_language_override =

На ось це:
[LANGUAGE]

;Leave this blank unless you want to load a custom locale file. The mod will default to your game language.
mod_language_override = ukrainian