Not sure it makes any difference to you been successful. Aguero seems to be doing just fine at Man City. Not done bad I don't think.
The players I've mentioned have all said it makes a massive difference, not necessarily on the field, but in terms of settling into life in that country, which in turn translates into how well you're likely to play. Speaking from my own experience, I have to agree.
You don't think that being able to have a conversation with your colleagues and being able to make friends with your colleagues would help you perform better at work? Imagine you going to work. Now imagine not being able to talk to the people you work with because you don't understand them