Language /
Polish was chosen the HARDEST LANGUAGE in the world to learn... :D [1558]
Contemporary German inflectional morphology PALES by comparison with Polish:-) Where German's four cases are often quite repetitive, the seven active cases in Polish differ from one another at the drop of a hat, rarely repeating! Although there is to be sure a certain regularity in certain noun declensions, the same can scarcely be said for the numerals.
Verbal "perfective" vs. "imperfective" aspects can also be a not so gentle challenge for the foreign Polish learner. Even in languages such as English or German which are governed typically by 'tense' not 'aspect', learning to distinguish their usage in Polish is sometimes enough to drive even the clearheaded among us to distraction:-) Mostly, this is because what English speakers see as requiring a completed action, for example, in Polish do not!
An analogy with German might be case governance. Even my advanced students would still have to sit and ponder why certain verbs which seemed like direct object (Accusative) actions in English, required the indirect (Dative) object in German, e.g. the verb 'folgen' (to follow) etc..
I continue to make mistakes in Polish aspectual distinctions.