I want become a lawyer but I speak with foreign accent.
Does it effective to be a lawyer in USA when my english language is not a perfect? Can someone give an honest opinion.

Although a lawyer ought to command a perfect language skills and there is no way to get around this fact, however, in my opinion, there are some legal specialties which one would consider as more demanding in terms of language proficiency and others which are less.

To the first category I would refer litigation, for instance - the way you speak at times is more important than the law itself. As regards the second category, some would argue that tax law lays there. Note, that there are some specialties in which an additional language is almost a prerequisite such as Immigration Law...

Although take into account that you are not the only one whose native language is not English and there more and more foreign born lawyers in the States and some of them even landed a job... :)

To wrap up - though it's hard to overestimate the importance of English language competency, the answer depends on specialty you choose to pursue...