Twig Translation String without Enabling Interface Translation
Our solution in the end was to create a new Twig extension called tr.

The class of the extension plugin checks to see if a 'translations' directory exists in the current theme. If it does, it reads the containing YAML translation files into an associative array (en.yml, es.yml, fr.yml etc.)

When {{ tr('source string') }} is called in the twig template, it will check the loaded associative arrays to see if there is a translation available for the current language and requested string. If there is it will return the translated value.

This works for us since in our case we're only translating a fairly short list of values - short strings and labels like 'Read More', 'Learn More', 'Subscribe' etc. The rest of our translated content comes from the content translation of the entities.

Hope this helps...