How can I access language currently viewed in Views Customfield: PHP Code?
The standard way to display text strings in Drupal is using the t() function. You should enter your text in English and it will be translated to the corresponding string in the current user's language.

If you still want to know the current user's language, you can load the user like this:

$user = user_load(uid);


and the language will be available on 

$user-&gt;language