Hello 
I am not sure this is maybe possible now somehow. I didn’t find a solution yet. 
For now we have this setting
allow user locale
When we turn on allow user locale it activates a dropdown on the user preference interface page which contains the all languages.
I think it would make sense to add the ability to select only those specific languages which needed. I mean for example if we have an international site then we can activate all languages but if we only offer the site for two countries or languages then we prefer to only activate those two languages and then as it expands we can add more.
Other setting
set locale from accept language header
For example: My site default locale is English and I want to activate one more language, Hungarian. In this case this site setting, set locale from accept language header will set the language automatically to Hungarian if they locale Hungary otherwise the language will set to default locale.
Thanks 