And to add to that, local administrators will distribute the software with the default option, that most certainly picks a translation locale, an unwanted effect when the option of language is unclear overal and the software is suited to programming (a task mainly done in English).
At this point I'm not sure which problem we're solving? Seems like it's now more about _"On Windows, let a user have Geany UI in English, instead in local LANG"_.
Indeed, if the user has permissions to install, then not installing the language files will do that. However, such user is free to administer the installed Geany in any way wanted, shortcut fix, environment etc. On the other hand, if it's a multiuser installation (the end-user has no admin permissions), then this blanketly forces all users into non-local UI.
I think, the middle ground for such rehashed issue could be to auto-generate (on Windows) an __additional__ shortcut specifically for English UI. That shortcut will have all the needed "secret-sauce" as described in FAQ. Basically, we're talking about updating the Geany Windows installer, perhaps adding another option on Windows, or detecting non-English locale and auto-generating the additional shortcut.