If you've followed the steps to setting up and working with the POEditor plugin described here and you still can't see the translations you have in your POEditor project displayed in your WordPress instance, you can check for the following:
1. You have the cURL library enabled.
The plugin uses cURL to communicate with the POEditor servers. It is mandatory to have this library enabled on your server.
2. Each language in your localization project is connected to a .po file in your WordPress language directory
Go the the POEditor plugin and make sure that each language in your POEditor project has a corresponding language file assigned to it. To do this click on 'Assign file' next to the project-language pair and select an existing .po file in your WP language directory. If such a language file doesn't already exist, create one, filling in the 'Add new' field in the appropriate location. It is important that the files are named correctly (see point 4).
3. You've imported the translations from your localization project
Go to the POEditor plugin and press 'Import' next to the assigned .po file to receive the translations from the POEditor project. This is necessary because the translations you make in your POEditor project aren't automatically brought to your WordPress instance.
4. You're using a localized .po file (not a .pot file) to display the translations
Go to the POEditor plugin and check that the file you're importing the translations to from your POEditor project is a .po file. If you've assigned a .pot file, it won't work, because the .pot format cannot contain translations, it can hold only terms.
5. Your localized .po file is correctly named
Check how the original files are named in the language directory and name all your other language files accordingly. The naming of the original files depends on the developer who created the WordPress theme/plugin, so we cannot offer a general example to follow.
6. Your WordPress is set to be multilingual
The POEditor plugin is built only to manage translations between the POEditor localization platform and your WordPress instance. It's not designed to also make WordPress multilingual. Once you've imported the translation to WordPress, how you set the language depends on each WordPress version. You either set the language from Settings > General > Site Language, or you edit the wp-config for older versions. More details here: https://codex.wordpress.org/Installing_WordPress_in_Your_Language