My doubt is: if I buy a localized European version of the game (example: the German one), will English text always be selectable as an option, or do I risk to have a game only in German?

I don't know if the earlier titles were ever localized into other languages. I know it started with Morrowind. Steam allows you change languages of installed games in most places other than Germany so that may work for Oblivion and Skyrim. Note, I don't know if Oblivion would have the multiple language option or not, but Skyrim does.