How to add translation for a string

I do not know direct answer on your question but this maybe helps you.
You can use plugin Loco translate or you can do it manually like this.

_e( 'Search', 'name_tag_of_translation' );

Then go in wp-content/languages and there define what is you translation, in your case we talk about plugin, so you go:
wp-content/languages/plugins and find your plugin translation or make new file. Creating new file is important, read more on https://developer.wordpress.org/themes/functionality/localization/

My example of translation:

_e( 'Search', 'shopkeeperchild' );

Path where is translation: wp-content/languages/themes/
Files which are responsible for translation:
shopkeeperchild-de_DE.mo /* Human readable /
shopkeeperchild-de_DE.po / MAchine readable */
For translation I used PoEdit.