Your browser language is the defining language for game launchers (e.g. Steam, Epic, Origin), as well as for Steam games that support the user's browser language. The language will be changed only for games that support this feature. For games of other launchers, the English language will be applied. The automatic language change is not supported in the desktop Boosteroid application.
To change the browser language in Safari, you need to change the language settings in the system. When you change the browser language for Firefox, you need to restart the browser before the settings would be applied. With Edge and Chrome, you just need to change the language settings in the appropriate browser and such settings would immediately be applied.