I wrote (and will write) on following topics:
Also I know some topics in biology and IT field but I believe that there are better than me and so I'll limit myself to edits if I find any errors. I will no longer write on Polish-German relations as there is really no sense in perpetuating revert wars with untoward people (please inform me if I miss any Wikipedia policy that is aimed at relieving this problem).
Also, I sometimes scan RC, keeping an eye for vandals and perform janitorial duties (mostly interlinking) on articles that desperately need it. I don't correct others' language however - frankly, my own contributions usually need them.
Recently I commited myself to translating English Wikipedia articles to Polish Wikipedia (and sometimes vice versa). I found out that while Polish Wikipedia is one of the largest in article number, it is severely lacking in terms of article depth. My own private theory is that the reason of such a state is that Polish readers grown up reading Spartian dry encyclopedias from PWN that were standard in PRL. Time will tell if this will change.