[Loki]

Beckett wrote some of his plays in French and then he translated into English himself.
It would be interesting to compare the two versions. I seem to remember that he wrote in French as the form would have been simplier: correct me if I'm wrong.

Then there are authors whose first language is not very well known and who decide to write directly in English to reach a wider audience; I can't think of any to be honest.

If you were a writer (maybe some of you are) why would you choose to write in a different language from your native one?