I can only intensively learn one language at a time. However it is possible to brush up on different languages especially when you're required to use it, such as in classical singing, for instance. Musicians involved in choral singing, operas, and other vocal art forms are required to at least understand the language they are using in songs as well as pronounce the words perfectly as required by the language. In the conversational level, most of us non-English individuals can handle two comfortably and use interchangeably during casual conversationa: our native language and a secondary one (usually English).