Thanks. It is all clear now. I’ll write it down for whoever is interested (ok, probably just for myself )
Transport Fever was apparently developed in both German and English. For the English version of the game, only the originally German strings had to be translated, so only those strings are in both .mo files, and for the German version it is the other way around. Other translations have both German and English source strings in the same .mo’s (see image below).
So I could convert the Italian mo’s to po’s, remove all translations and translate from there, but that would seriously mess up my translation memories (I don’t want German strings in my English to Dutch translation memory), so I will translate them separately and merge the .po files later.