Hi,
If you check the language file in the HikaShop configuration, you can see that the "board game" translation is done with this line:
JEUXDESOCIT="Board games"
So the key to use for the title is JEUXDESOCIT
So replace "Jeux de Société" by JEUXDESOCIT in the title of the custom field and it should work.
Note that this is the title of the custom field. On the filters area, the "Jeux de société" you see there is the name of the filter, not the title of the custom field. So if you want to change it there, you need to change the name of the filter with the translation key.