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Yes, you can use pre-created translation files. Please read the documentation first – https://xstore.helpscoutdocs.com/article/30-base-theme-translation
Download .po files from https://www.8theme.com/glotpress/projects/xstore/xstore/fr/default/
https://www.8theme.com/glotpress/projects/xstore/xstore-core-plugin/fr/default/ , rename them to xstore-fr_FR.po and xstore-core-fr_FR.po and upload into correct folders. Translation files of the theme should be placed in /wp-content/languages/themes/ folder,
plugins, like Core plugin, in /wp-content/languages/plugins folder, then you will not lose translation after updates.
Do synchronization – https://xstore.helpscoutdocs.com/article/103-translation-files-update and continue translation.
If you need our assistance, please provide URL of your site and temporary wp-admin access via the Private Content area.
Create an account via Dashboard (Users > Add new) with administrator role http://prntscr.com/s3rc9m > provide us with username and password via the Private Content area http://prntscr.com/s5mao7 You can use any email for it.
One standard license is valid only for 1 project. Running multiple projects on a single license is a copyright violation.