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Welcome to Steemit dude!

Why not both? Or maybe, whichever one suits the subject matter best in your opinion. I'll struggle through the french for good content.

I would say both ;)
If we want to attract more French people here, we need to publish more French content. Apart from that, I'd say write the post you want to publish in the language you prefer at the time of writing. Then you'll think about translating it if you want to get exposure to a specific community.
Juste mes deux centimes ;)