I have had this problem on and off and I haven't been able to find the reason. Word 2016 freezes when I change the language using custom defined short keys (Ctrl+1, Ctrl+2). More interesting that it won't freeze if I change the language from language bar (by selecting the language in notification area).
All I guess is that there is some wrong information in word files that cause this kind of freezing (might also be from conflicting shortcut keys).
Many people have this problem and Microsoft has not given any solution. The problem is mentioned in these questions: