AFAIK the term Slavic is simply a designation of a language group.
So, there are two possibilities here about why your friend was so quiet. The first possibility is that his quietness is simply your over thinking this and had nothing to do with your comment. There are dozens of other reasons why he may have been quiet, not th e least of which being that he simply didn’t have anything further to say and isn’t the type to make inane small talk.
The second possibility is that your perception is accurate regarding the quietness. But it may well be due to the reasons cited by others above.
Really, the only person who can give you an accurate answer to your question is your friend himself. We have no idea what was in his head.
Ask him directly. He won’t mind you showing concern for his feelings. And you’ll get an ACCURATE answer since he’s really the only one who can tell you what’s going on inside his head.
And his answer might surprise you. And you will have learned something important regarding your friend.