I18n config option for locale on single-language sites#246

This was once discussed in the old Ideas repo and addressed in a subsequently postponed draft PR:

Currently, if a single-language site’s default language is other than EN, plugin UI output is rendered in English even if a suitable translation exists.

While a workaround is possible (in the linked Idea issue), it would still be great if plugins could display their UI texts based on a global locale without the need to override the I18n config on a template basis.