Languages are saved in /public/locales in JSON files. These files need to be in sync with each other so every change made to one file needs to be added to every other file too.

<LanguageSelect /> is located in /src/modules/Settings/components/Interface and will automatically set a cookie for the new language and refresh the page to apply the changes. LANG_DATA array stores all available languages just make sure that value prop matches with {value}.json in the public folder.