You usually need the definite article with languages: étudier LE français, apprendre LE chinois, comprendre L'allemand, enseigner LE portugais, etc.
The verb "parler" is a special case and the article is dropped in most cases by some native speakers (if you specify the language, you cannot omit the article: parler LE français DES ANTILLES, for example).
If you are talking about people who are USING a language, SAYING things in these language, I would recommend you not to use the article (it sounds more natural for me at least). For example: "those 3 students over there are speaking Chinese or am i mistaken?" = Ces 3 étudiants sont en train de parler chinois ou je me trompe?
There is a long thread about this topic in Word Reference...
The day-to-day use of the article depends though on lots of things: if French is the mother tongue of the person, if the language is well known, if you speak other languages. It depends also on the speaker =P