I don't see what config setting would cause this - so why not disable your custom config settings one by one, reload the cms, see what gives?
Okay tested few things.
1. Locale in CMS is set to pl_PL, it's visible in templates
2. even though it's set correctly _t() elements are not translated
3. i will test translations by deleting modules
After 3. i got to point where i know that either Event Calendar plugin or dataobject_manager cause configuration problem.
now i have to find where exactly problem occurs.
I would now check if both modules have a Polish language file. If not, afaik they will not automatically use en_US.php, but instead use the default, at least for those two modules...
If a polish file exists, check for php errors. try removing them, and see if that will make the rest translate...
I found the problem.
While configuring Even Calendar i included pl_PL lang in _config.php that cause the problem.
Now everything is back in place. HOORAH.
Thank you for help. In the end it was my fault.