You will write what you "hear" in your head, so I think you may just need more practice using and hearing the language yourself. Part of the complication is that you are probably having to think about sophisticated legal concepts in English, and I imagine that this is challenging if you've only recently learned the language. I encourage you to talk to the people at your law school (maybe an advisor for LL.M. students if you aren't an LL.M b/c they're probably used to helping foreign attorneys with their English language skills) to see what support there may be for you.
As far as general language practice -- Have you considered taking a formal ESL course (but maybe an intermediate or advanced one) to help you become even more confident in your English skills? Also, if there is a local Spanish/Portuguese cultural group in your area they may have an "intercambio" program that would pair you with an English-speaker trying to learn your language. You all could help one another. Finally, what I find helps me maintain my foreign language skills is listening to foreign language newscasts (notice I didn't say "watching" them b/c I find that too distracting. Instead, I'll just turn it on usually, divert my eyes, and concentrate on really catching the words I'm hearing) and reading *aloud* the free foreign language newspapers you can pick up around town. I find that the language by print or television journalists/reporters tends to the standard usage and, thus, easier to understand than the slang spoken on the street. I still think it's helpful to make the effort with these sources though just to train your ear to hear how native speakers talk and write.