add french translation for search form placeholder

[fakenine]

Hello,

This adds a missing translation for the "Search Messages" form placeholder.

[EpokK]

Translations are done on Crowdin. More info here: https://protonmail.com/blog/translation-project/ https://protonmail.com/blog/translation-instructions/

[fakenine]

Hello! Ok, thanks

[mKoonrad]

@EpokK What can I do (as translator) when translations are ready (for many months) but your front-end team don't update it on web? I know that localization isn't probably you main priority but after many months and main strings you should fix it.