I've just been pointed in the direction of empik.com and, converting the prices, I've spotted some pretty good deals on there in comparison to German prices. I know that non-Steam games are a bit of a gamble there as to whether they have support for German or English, but how does it stand with the Steam games? Are the keys in Poland classed as ROW or are they specific to the Baltic countries (i.e. not activatable here in Germany or are only playable in Polish, Czech, Russian etc.)?

Dunno about activation, never checked.

When it come to language, all games on steam that I have (PL versions) are playable in english.

Yeah, what he said - I think steam keys are not language locked, you pick the language of Steam itself and may be abe to choose different game languages on some specific products.

For example I have Orange Box from Portuguese retail, but after registering on Steam I never had any issue with playing HL2 or Portal in English.