Wordpress Site Language change not possible

I installed a Wordpress and imported the content from source site which I try to renovate now.
I noticed that Site language is English (United States) now.
If I change it to local language, files get installed into /domains/myDomain.com/public_html/dev/wp-content/languages and the language does change to local language but the field Site langueage remains on English (United States). Why?
That wouldn't bother me much but I cannot use Loco Translate to translate some Theme words.
What could be wrong:
Thank you.