How to change default language Chromium/Firefox (Playwright)

Hi guys,
I’ve been trying without sucess to change default language of browsers Chromium and Firefox (my tests run in parallel) to french language. For Chromium, I’ve tried to use --lang flag without success and I’ve also tried to use prefs. For Firefox, I’ve tried to use firefoxUserPrefs. Till now, nothing has worked.
Does anybody know how to change default language?

CodeceptJS version 3.0.6
Playwright version 1.10.0
Chrome version 90.0.4430.0
Firefox version 87.0b10