You can write it in any language you like, whether it is English or French or Portuguese or Russian. You don't need a specific level of English to write raps in general. However, if you want to write a rap in English, it helps if you know enough to be able to rhyme and make your meaning clear. Maybe check with a native speaker if you are unsure.
Depends on your style of rap. Still, it helps to have a good understanding of language before you go writing your rap - including slang. Some advice - before you go writing anything, pick a style. Being new it may help to pick out some distinctive rappers and take a look at their songs - don't just listen to them, really pick out the lyrics, see what makes it good. You should notice themes. Never start writing lyrics without picking a theme - unless you're very good, it doesn't work!