Translation of theme and plugin .po language can easily be made right in the WordPress dashboard. Loco Translate reads the .pot template file, provides you with a solid user interface to locate and translate strings, and compiles the .mo files for you. Sounds confusing. It really is not.
Demo
For this site, I’ve added a left and right arrow to the UK English version of the Previous and Next Post buttons below.
Extract from WordPress.org
Loco Translate provides in-browser editing of WordPress translation files. It also provides localization tools for developers, such as extracting strings and generating templates.
https://wordpress.org/plugins/loco-translate/