Bekir, that is the way Oxwall was originally developed. Users can switch back and forth between languages and we can't track which language is being currently used.

You can suggest this new feature ( choose language of email notifications) at UserVoice

I agree with Bekir and I give my votes to uservoice.
Is there a possibility to see which language use my members?

No, there's no such way in the default software. 

I believe this is a custom plugin development. You will need to hire a programmer to do that for you. You can do that here:http://www.oxwall.org/market/specialists

Hi Ross,

Would be nice if Oxwall took the responsibility to have the choice of user language as default but still having the translation as an option by the users itself :). This way Oxwall will put itself as a native international platform. :)

I know about: uservoice. And I am saving to donate ;)

You can add your comment there. 

Voted on uservoice as well.

This is a serious issue. As we just realized all multilanguage is useless because our visitors do not join the community as they simply do not understand what is written in the email confirmation letter.

In oxwall you may simply "save" the last language user has selected and use it for emails. This is a very easy fix if you have an access to core.