It is actually an elliptical sentence. The elliptical part that is "的原因". So the completed sentence should be 我想学习汉语的原因是因为我有很多中国的朋友。 Example: 1.我想去美国学习（的原因）是因为那里有全世界最好的大学。 2.我想学钢琴（的原因）是因为我看了一部关于钢琴的电影，从此便对此有一种特殊的感情。 For this kind of structure, you can also divide it into two sentences. 1.我想去美国学习，因为那里有全世界最好的大学。 2.我想学钢琴，因为我看了一部关于钢琴的电影，从此便对此有一种特殊的感情。

I can't explain this, maybe it's just the way we say it. And you can replace "是" with a comma, so "我想学习汉语是因为我有很多中国的朋友" "我想学习汉语，因为我有很多中国的朋友"both are correct.

to add certainty

Actually the 是 makes a significant difference to the meaning of that sentence. It implies that, prior to this sentence, you HAVE told the listener that you want to learn Chinese.

it is for making an enfaces on the reason. Is something similar when in english you say "I like" or "I do like". Maybe the context of the sentence needs to emphasis that is this reason and not that other reason.