Extensions/Language Files may be deleted during an upgrade/upload based on how your FTP program handles the upgrade/upload process.
Different FTP programs handle this differently. We recommend you backup your WordPress database including any existing extensions and any language directories (if you use different languages on your site) before you upgrade WishList Member.
For example, say you copied your translated file into WishList Member “lang” folder. If you delete WishList Member, you will delete that folder too.
So you will want to make a copy of WishList Member “lang” folder and copy it back to same place after upgrade. Note that this is typically only needed if you upgrade in the WordPress admin area.
If you use FTP to upgrade then you can overwrite the new WishList Member over the old WishList Member folder and lang folder will remain untouched.
All that being said, we highly recommend you do a full back up before you upgrade just in case.