Hello,
1. It is not possible to change the language on some pages and leave the rest of the site in another language. You can create and insert the content in any language, at the same time, all the functional strings would be in the language you have set in the Settings > General.
Please make the full backup of your site and check the issue on the Storefront theme to detect if our theme causes the problem. Storefront is a free theme developed by WooСommerce.
Best Regards