Can not change language

I have a multisite network and have a few sites. In one of them I can’t change language. I only have two languages installed (English and Polish). On every site I can go Settings -> Site language -> and select language, then click Save and that’s all. In one site I set Polish, click Save and I see info “Settings are saved” but in language field I still see English. And my site is partly in English abnd partly in Polish. For instance I see “Submit comment” instead of “Napisz komentarz” (in polish. I tried to save manually in site settings pl_PL and this setting is saved but still see something in English on this site. On the other sites changing language works perfect.