Firstly check out the language descriptions:
Top Sony - Languages Description
Pick which ever you want

Headover to the download section
Downloads - R8AA018_RED52 - Top Sony Forums
Download the GENERIC MAIN and EUROPE 1, 2, 3 or 5 because all four contains French.

You will then need to download the custom file:
Downloads - k770custpack - Top Sony Forums
and download which ever EUROPE you downloaded for FS.

Use XS++ to flash the phone with that firmwire:
XS++ 3.1 Guide-Flashing,Customizing,Recovering - Top Sony Forums
and then start FSX and put the custom file into:
;Custpack -> 'TPA/PRESET/CUSTOM'