Steps to reproduce the issue

Install French fr-FR language
Create a new override after setting Language and Client to
English (United Kingdom) - Site
Search for a constant OR a value
JYES or Yes will fit

Actual result

No result or wrong result (Only the SampleData testing plugin will give results as strings have the en-GB prefix)

Switch Language Client to French

Search for Constant or value
JYES or Oui

It works fine.

Why?

This happens since the language prefix was taken off from en-GB.

@Bakual