Update fr.json

[ygranger-oa]

Hi,
I'm a french guy, and here my corrections for the focalboard plugin. I didn't find the focalboard plugin in your translation server. So I post my PR here.
Sorry if I don't respect the normal behavior.

As soon as I will get more available time, I will take a look of the french translation in the mattermost section.

Yannick

[mattermost-build]

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[mattermost-build]

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[ygranger-oa]

/check-cla