Do you have to be extremely good at English to become a lawyer?Watch
But you do have to be able to communicate very clearly, calmly, have strong comprehension skills and be able to cope under pressure.
As you are 14 then I would suggest you have time to improve your English, increase your vocabulary and broaden your reading skills.
Plenty of time to improve.
Read broadsheets, watch the news, read books. If you come across a word you dont understand look it up.
Quite a few of my friends who did Law at Edinburgh disliked studying English, but they were competent enough to get a decent grade. They mostly did stuff like History, Religious Studies etc at A levels instead of English.