'Brummie' Accent Concern for French Expat!

They have variations of that accent pretty much across the Midlands.  You'll get used to it.

Brummie is a dialect, just as Scouser (Liverpool) is. Some will speak it harsh, some soft, some quiet, some loud, just like any other dialect/accent. It is the same with Irish and Scottish, although even the English have difficulty understanding the latter I would just ask the Brummies to speak a little slower if I were you, just as I would with any other language I might not understand or hear clearly.

I haven't came across someone leaving a place solely because they couldn't understand the accent. Not saying it doesn't happen, but it seems nearly all manage to fit in one way or another without a problem. I went to Birmingham a few months ago with my brother. I struggled, but he was totally fine with it. A bit more time, I would also be fine with it. Some come to the country with little to no English and do well. You're already coming with fluency, so I don't see any issues at all.

Good luck.