The language I use was incomplete. That’s why I used the .pot and .po files within the plugin and completed the language through Poedit. I’ve moved the files to /wp-content/languages/. To no avail.

Then I went to translate the language online. I’ve finished. Then, export: .po, .mo, and .php files. Renamed. Put it in /wp-content/languages/ and everything works perfectly.

I didn’t have time to research what the problem was. The problem is with the language files within the plugin. The language files for translation online are fine.

Maybe my experience will help someone.