Save chosen language

This sort of thing is pretty much exactly what PlayerPrefs was designed for. I assume you have some sort of value that identifies which language the player selected from the menu? That's the value you would save in PlayerPrefs.

I would create a function that enables the selected language:

 void SelectLanguage(int languageId)
 {
   // do the stuff to enable the language based on the language id
 }

When the user selects a language from the menu...

 int languageId = the id the user selected from the menu
 SelectLanaguage(languageId)
 PlayerPrefs.SetInt("Language", languageId);

And on startup you need to also enable that language

 void Start()   // one some game initialization object
 {
     int langageId = PlayerPrefs.GetInt("Language");
     SelectLanguage(languageId);
 }

This is mostly pseudo-code so it may or may not work as is. The point is, create a single function that enables the appropriate language, and call that same function from both the menu when the user changes the languages, as well as during startup.