Updating store_lang (English) with 1.6.4 strings

Updating the English store_lang.php with the strings provided in Store
1.6.4
Updates to the French language file store_lang

Made minor changes to the French store_lang.php file.
Added the new string introduced with 1.6.4
Please add the French language folder in the basic install of Store !
(we can find all languages except French that has to be downloaded from
Github !!)
Thanks
Merge pull request #8 from ebeauchamps/master

Updating store_lang (English & French) with 1.6.4 strings