My own user case is that I am not native English, but prefer to have all my software in English, including OS.
Yet very often newly installed software seems to source ‘location’ for it’s user interface language, and not the selected language.
And that’s what Picard seems to be doing (or being mislead by) also.
edit,
For Windows it’s probably sourced from ‘Region’ > ‘Formats’ > 'Format:'
Many ‘foreign’ users will select English as OS language, but will want to set their currency, number formatting, dates, etc. by local standards.