I am preferring to watch football on the television without artificial crowd noise when I can. This means that it is often possible to hear the managers and players. I was watching Arsenal and Fulham and heard a few shouts in French as well as English.
The rather old fashioned pundit Martin Keown was disapproving and said that English should be the only language used. This seems to be an assumption but as the manager of Arsenal, Mikel Arteta is multilingual, it seems reasonable that he should make use of his linguistic capabilities to speak in the language that is the most appropriate for the circumstance. If he is able to speak French to the French players, it may give them a very slight advantage in terms of the time it takes to process the message as it is in their first language even if their English is good. It might also help them affectively in feeling that their first language is valued.
The use of French might also be of value in confusing non French speaking opponents. I remember listening to an interview with the ex Coventry player Dave Bennett where he said that he and Cyrille Regis often used patois, partly to confuse opposition players. It might also have helped to bond Regis and Bennnett through commonalities in their backgrounds..