Let me just preface this by saying I always start in lower leagues and try to work my way up, so my experiences are usually from about 2013 or later since I don't have the finances to bring in many foreign players. When I start bringing in players from other countries, I always try to have a coach or another player that is the same region as them (i.e. Scandinavia, Eastern Europe, Africa). These coaches don't even necessarily facilitate any of their training regimen or anything. I am probably just imagining this as superstition in my head, but possibly there could be something with players fitting in based on region as well as language. I am in 2024 in my current save and I have only ever had one player have problems fitting in (an English regen I brought into my Spanish club).