请问在英语中，为什么这么多外来词汇？如法语词汇cause célèbre，为什么不能用英语自己的词汇呢？

一些词汇不好直译，所以就要用原词来代替，否则容易误导。

shida258说得没错，学学英国历史就知道了。
转自wikipedia：
Old French (Anglo-Norman) was the language of the legal profession in England, starting about 200 years after the Norman conquest in 1066 (the years 1275 – 1310), to about 1731. Some of the Old French used at that time (Law French) remains in use today as English pronounced Anglo-French words: "appeal, attorney, bailiff, bar, claim, complaint, counsel, court, defendant, demurrer, evidence, indictment, judge, judgment, jury, justice, party, plaintiff, plea, plead, sentence, sue, suit, summon, verdict and voir dire." While there are many French-looking words, they may not correspond to Modern French (for example, "voir" in voir dire is a completely different meaning).[3]

学学英国历史就知道答案了。