Fix and update French translation

Fix fr.json (french translation) and update it with the new strings from en.json

closes #920
- adds & translates lines 138 to 154 from en.json
- fixes spelling, grammar and phrasing errors
- fixes vocabulary for constancy
- adds spaces where needed (in French it is not "Word:" but "Word :")
- fix string 149 to fit the actual content and translate it (should be done in en.json too)