Van Gaal, who guided the Netherlands to third place at the recent World Cup, has been tasked with ensuring United bounce back from a torrid campaign under David Moyes last season.
United finished seventh, their lowest-ever Premier League placing, and missed out on European qualification.
Van Gaal has players from a number of nations at his disposal, but expects them to use the same language during matches.
"We speak in English," Mata told BBC Sport. "The manager says we have to speak in English."
Mata also made reference to the change in style that has been implemented by Van Gaal as United look to return to winning ways.
"The truth is we are trying to play with a new system," he said. "I don't know how much time it takes - maybe one month or two but the most important thing is to believe in what we do.
"For me personally if I play behind the two strikers it's a good position. I can look for the ball. I feel comfortable in this role.
"This system can be very dangerous. But you have to communicate and co-ordinate.
"This club was made to win. Ever since I was young Manchester United was winning trophies or fighting for them. That is what we have to do."
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