Plugin updates replace the entire plugin directory. Any customization, whether they are PHP, CSS or language files, will be overwritten.
Here’s the proper way to customize your language files. (If I can find this documented somewhere, I will post a link later. Maybe the moderators can help?)
Create a languages directory under wp-content:
/wp-content/languages
Create a plugins directory under that:
/wp-content/languages/plugins
Place your custom po/mo files in that directory. They can be either copies of the po/mo files provided in the plugin or they can be new files you create with only the phrases you want to translate.
/wp-content/languages/plugins/strong-testimonial-nl_NL.po
/wp-content/languages/plugins/strong-testimonial-nl_NL.mo
WordPress will load these after the plugin’s translation files so your customizations take precedence.
This also works for themes, by the way.
