Overrides are meant to make small edits. The interface isn't designed to be completely translating it into a new language and can cause your to hit server input limits. Have you considered contributing to the pl-PL language instead? Details on how to do this and to join a Transifex language team can be found below.

www.joomlapolis.com/localization

Is this a bug in the interface?

No, you're probably just hitting server input limits. The fix is to increase your server input limits within php.ini. Regardless it's still not meant for completely overriding every string. In a future release we'll have improved storage behavior for overrides to avoid hitting input limits (this is usually done by just converting to JSON and sending the JSON object on form submit).