Most likely one or several of the language files of the other language are damaged. It might look alright but maybe there's still for example an encoding issue in at least one of the files or an error in one of the language tags included.

Have you installed only the language files of the core Elgg system or any additional language files of 3rd party plugins? Also, Tidypics itself comes with several languages supported already. You could temporarily remove the language files of the other language and then add them again one at a time to find out if the error is due to one file only or if it's a general problem with this language.

iionly:   Thank you so much for the suggestions. I have recreated a few language files and now it works! Indeed, new files look exactly the same as previous ones, however an invisible encoding issue was the problem. Thanks again!

Uservalidationbyemail is disabled. I created user accounts using regular Elgg 1.7.5 "Register" and the registration was confirmed by Elgg temporary message. Then I'm trying to login with username and password, and coming to the same (but empty) login page.

Elgg 1.0-1.7 requires some sort of user validation. Beginning in Elgg 1.8 this is no longer required. We added manual validation to the uservalidationbyemail plugin in maybe Elgg 1.7.6.

You could create users in the admin area (User Administration) and they are automatically validated.

Ok. As I understood, I have to use Uservalidationbyemail and it cannot be disabled in Elgg 1.7.5. Everything works now. Thank you!!!