Mixing up languages

I was on a trip to Germany recently visiting a Serbian friend. I thought I might have some fun and try to get around with speaking the language, but now I accidentally put a lot of Serbian words in the German sentences (Ich möchte ein kaffee i pivo, lol). I pretty much completely butchered it.

My German felt fine on my last trip here two years ago, but now I had some real problems using it. For reference I haven't actively learned German since high school 10+ years ago, and I am currently learning Serbian, where I am probably close to B1. I was at a decent level in German, and I still understand a lot because it is similar to my native language.

It doesn't bother me, and I don't feel embarrassed over making mistakes when speaking, but it got me thinking. Can this mix-up happen if I decide to learn another language as well? A friend has been interested in starting learning Greek with me, and I am also interested. But I have some concerns that too much language can "break" the other ones.

So what happened here? Too little use of the language over time, or were me and my friend simply hitting the weissbier too hard during the days I was there?