When an extension tries to use email templates (ext/vendor/ext/language/en/email/template.txt) and that template is called via

$messenger->template('template', 'en');

you get an error (file not found) because the messenger only looks in phpBB's language folder, and not within extensions.

It needs a way to know to look at ext/vendor/ext/*
Something perhaps like with languages,

$messenger->ext_template('vendor/ext', 'template', 'en');

?