It is often easier to write the first draft in your native language. It allows you to put in all your nuance. Afterwards, you can translate it into English. Inevitably, you want to have a native speaker of your target language (English) to read and correct your script. If you choose to write in English, be aware that there are still challenges.
Since language is so closely aligned to culture, it can often make exact translation difficult. And in some cases, it changes the intended meaning. I had a friend who said when she argued with her husband, she used English rather than Korean. As the Korean language imposes on women a more deferential vocabulary. Slang, the informal language of everyday life, is often a challenge. While I am often impressed with the knowledge of many non-English speakers to swear and curse (probably from watching English language films), there are many common aspects that are missed. ("Whaddya mean it needs fixed?" = "What do you mean it needs to be fixed?") Even speakers of the "same language" can have different spelling and expressions, which sound odd to the other.
Humor is the most difficult aspect. Humor is so specific to a culture and language that it often doesn't translate easily. Sometimes it is very subtle. Puns (words with slight changes that dramatically alter the meaning) are very language specific. Sometimes it is the simple typos that make situations funny. Scheiss for schiess, connasse for connaisse, etc. Sometimes words have many meanings so it often requires finding the right word. A 'comrade' in English is friend, but in Russian 'tovarich' is more of a greeting like 'citizen'. So a 'comrade in arms' (close friend) might become 'tovarich po voyne' or 'war citizen'. Even situations that are funny in one culture can be viewed as offensive in another. It takes someone familiar with both cultures to give that feedback. While having the clothes ripped off someone is often viewed as funny in European cultures, in Middle Eastern cultures that can be highly offensive or be seen as a punishment not amusement. While many think that English speakers, especially Americans, are not easily offended, there are times when it happens. Jokes about the attack on the Twin Towers are still in very poor taste.
I was invited to help write a script for a Spanish language production. I wrote the first act in Spanish and passed it to my friend. I found I couldn't provide the same nuance I could in my native English. His feedback pretty much confirmed what I felt. Rather than spend more of my time translating nuance by thumbing through my dictionary, I re-wrote everything in English. Afterwards, we focused on making the translation. It also required that I learn much more about Mexican and South American history and cultures for this period piece.
The most important step is telling your story as you imagine it. My suggestion is to write it in your native language first, especially if you find you spend more time using the dictionary than writing. Later translate it and work with a native speaker of English to refine the script.
While I don't want to discourage submitting scripts in English, I think developing film communities in one's own countries should be encouraged. With the internet, you might want to see who is close by that may be interested in collaborating with you. Today's technology makes filmmaking much more accessible.