Hello,
What is the language of your site (Dashboard > Settings > General > Site Language)?
Install Loco translate plugin, you will be able to disable the plugin once translation/changes will be done. The plugin is required to sync existing translation files https://xstore.helpscoutdocs.com/article/103-translation-files-update It is not enough to just place files to the folder, go to Loco translate > Themes > XStore > open translation of your language > sync translation > find and translate strings > Save changes. As was mentioned “Categories” should be changed via translation of Core plugin, so go to Loco translate > Plugins > XStore Core > create translation to your language (languages/plugins folder) > http://prntscr.com/qvg90j
If you need any further assistance, provide URL of your site and temporary FTP access.
Regards