Why does one say "je vais parler un peu français" or Je parle un peu le français when un peu is normally followed by de. Even if languages are usually

Bonjour Ann !

Have a look at the following lesson:
https://french.kwiziq.com/revision/grammar/using-le-la-les-with-titles-languages-and-academic-subjects-definite-articles

Also there is a difference between "un peu" and "un peu de" in French, the same way as "a bit" and "a bit of" in English.

In this case, he's saying "I'm going to speak a bit French", i.e. "I'm going to speak French a little", hence "un peu".

I hope that's helpful!

Bonne journée !