But when playing with our Slovenian group, it was easier for GM and the players to use English - Tenkazu can say more about that as he was the GM in question :)
Most people from my generation started learning English in kindergarten. I don't know how it was with previous generations, but today, it is sort of expected anyone knows English basics at least. People who drop English in favor of German or other language, through their kindergarten or elementary school, usually have problems later at catching up with others, finding the right literature or have some other problems because of their choice. Most jobs nowadays require of each applicant to know English, and sometimes more than one language is required.
As far as the games go, they're alright. I had the opportunity to play at English-only table hosted by our guest from USA for Slovenian visitors and me, and to run one myself just two weeks ago for the same Slovenian visitors. I can say both games went perfectly well. Save the guest from USA, we were all using our second language and everyone had plenty of patience and didn't mind the obvious mistakes because the basic idea was transmitted and fun was had either way.
Playing in English actually helps a bit with expressing certain things that would otherwise sound awkward in Croatian or can't be translated properly. Sometimes we throw English quotes at each other or express ourselves in English, even if the game is in Croatian, because we want to make a statement, for better immersion, because of the moment, etc. Main reason is that English sometimes sounds so much better or is more appropriate to the situation. Also, we're all quite tolerant so any communication problems eventually get worked out and if someone is having trouble finding a right word, others are here to help.
I don't have many opportunities to practice my English skills in general, lest so the vocal part but I tend to think I'm doing fine and the game with our Slovenian guests kinda confirms that (because I did most of the talking). Written English and passive understanding are other matters that we're basically good at, it's just the vocal part that gets us bogged down sometimes.