Ok this is an easy question except it isnt actually PHP related.
Why isnt it PHP related?
Ok PHP is a server-side language the reson its that is because all the work is done on the server. The only information PHP can get from a client is from headers majority of headers do not contain any information on what language the users Computer is using. However you may find $_SERVER['HTTP_USER_AGENT'] to be useful but it may not work with all browsers. that is get the language.
Ok now that i got that. I dont actually think there is away to get the users computer language even using JavaScript (you may want to prove me wrong and find something). So its not going to be likely you will be able to.