WP translation does not work

I upgraded my WP to the latest version. Prior and after the upgrade, WP stayed in English even though I’ve got the translation file and the config-php set to display WP in a different language.

Problem started when changing server (including database copy/restore). I’m told database has nothing to do with it, but I still can’t figure out what the source of this mess might be.

any help would be much appreciated.