Can someone express themselves through writing even if English is not their first language?

Answer: Certainly. They can write in their first language.

Or, if English is their second or thrid language, they can learn that well.

Most Europeans speak in write in multiple languages. Most Nigerians and Indians do too.

And Multi-millions of Chinese write better English than most native English speaking Americans do.

At least half the human race has a good innate ability to learn languages, both spoken and written. The rest is simply application and effort.

Hope this helps.