I fixed it by editing line 4445 in codestyling-localization.php
from […](!csp_pofile[csp_idx.cur[i]].val.empty() ? […]
to […](!csp_pofile[csp_idx.cur[i]].val[pl].empty() ? […]
I don’t know if this has any effect on other languages but at least I can use the navigation now.
If I look at the contents of ‘csp_num_plurals’, it gives me “1” in Turkish and “3” in the other languages. Shouldn’t this be the same? (The theme is the same in all Languages and there are 3 Plurals on the site)