i18n file for French locale

[gxolin]

Is your feature request related to a problem? Please describe.
Having a french version of the sheet will improve playability for players not well versed in english

Describe the solution you'd like
add src/lang/fr.json (I don't think it's worth to make an extended language subtag like fr_fr will be worth)

Describe alternatives you've considered

Version
beta ruleset

Additional context

Checklist:

• [x] I have searched the existing issues to ensure this feature has not already been requested.

[gxolin]

I'm working on a fork to implement this, I'll make a PR when ready