FAQ: How do I translate posts / pages / taxonomies / settings?
Translating content stored in the database is not what this plugin is for.
The text in your blog posts, pages, taxonomies - and even site settings - is all held in the database. This plugin cannot translate content in the database. Either use a different plugin designed for translating database content, or consider running WordPress in Multisite mode where each of your sites is set to a different language.
Loco Translate is a PO file editor
WordPress reads translations of static text from MO files which are compiled from PO files. Generally this means hard-coded strings in themes, plugins, and the WordPress core. If you're not familiar with PO files, try reading the WordPress documentation:
Translation or editing of anything that isn't a PO file is beyond the scope of our support.
Watch out for theme/plugin settings
Some plugins and themes allow you to configure custom strings within their own admin pages or customizer interfaces. In such cases the string will be saved to your database as a WordPress site option and so not editable via static files.
However, sometimes in this scenario the original strings may still be present in PO files. What tends to happen is the strings are translated only when the customizer interface is presented with default values. As soon as you submit the form the strings become saved in the database and are never translated again.
Critically, WordPress sites are not multi-language. Each site has exactly one language and so anything saved in settings is presumed to be in that language. A common way to solve this problem is via WordPress Multisite whereby each site has its own localized copy of the settings.