For such language tickets please give more information. What are your language settings? Not only in Kleo, the system language settings, too.

You could also try to delete klanguageoverridesrc and check again.

The screenshots were made with

• ISO: EnglishInternational
• Windows Language: English (United Kingdom) - note: i installed English (United States), but can't select it
• Country or Region: German
• Regional Format: German

The translation of GnuPG System in Kleopatra (as well as the output of gpg CLI) is in English after setting

• Regional Format: English (United States)

So i guess, this is expected? Although a bit unexpected, as the "regional format" description says, it's about "numbers, currency, date and time"

I have no idea how Qt/KDE and how gettext (resp. gnupg's replacement of gettext for Windows) react to Windows's "regional format" setting. It seems that Qt/KDE correctly use English despite German regional format while gnupg uses German.

It's not clear from your comment whether gpg CLI also used German with the settings the screenshots were made with.
Edit: I guess https://dev.gnupg.org/T7886#207284 answers my question.

Notes to self:

• On Windows, libgpg-error's gettext replacement uses the value of LC_ALL, LC_MESSAGE, or LANG (in this order) if set. Otherwise, it uses Windows's GetThreadLocale. (gnupg should probably use the MUI API instead.)
• We should probably force Qt's/KDE's language on gnupg by setting LANG.

Right, gpg CLI output depends on it, too.

The language settings of Windows have strange influence on Kleopatra and GnuPG.

Windows has:

• The system language (of the installed Windows)
• The Windows display language ("Windows features like Settings and File Explorer will appear in this language.")
• The Preferred languages ("Apps and websites will appear in the first language in the list that they support.")
• The Regional format ("Windows formats dates and times based on your language and regional preferences.")

Tested on a German system:

Windows display: en_US + Apps & websites: en_US(:de_DE) + Regional format: en_US -> both use en_US

GetUserPreferredUILanguages: QList("en-US")
QLocale(): QLocale(English, Latin, United States)
QLocale().uiLanguages(): QList("en-US", "en-Latn-US", "en")

Windows display: en_US + Apps & websites: de_DE(:en_US) + Regional format: en_US -> both use en_US

GetUserPreferredUILanguages: QList("en-US")
QLocale(): QLocale(English, Latin, United States)
QLocale().uiLanguages(): QList("en-US", "en-Latn-US", "en")

Windows display: en_US + Apps & websites: de_DE(:en_US) + Regional format: de_DE -> both use de_DE

GetUserPreferredUILanguages: QList("en-US")
QLocale(): QLocale(German, Latin, Germany)
QLocale().uiLanguages(): QList("de", "de-Latn-DE", "de-DE", "en-US", "en-Latn-US", "en")

Windows display: en_US + Apps & websites: en_US(:de_DE) + Regional format: de_DE -> both use de_DE

GetUserPreferredUILanguages: QList("en-US")
QLocale(): QLocale(German, Latin, Germany)
QLocale().uiLanguages(): QList("de", "de-Latn-DE", "de-DE", "en-US", "en-Latn-US", "en")

Windows display: de_DE + Apps & websites: de_DE(:en_US) + Regional format: de_DE -> both use de_DE

GetUserPreferredUILanguages: QList("de-DE", "en-US")
QLocale(): QLocale(German, Latin, Germany)
QLocale().uiLanguages(): QList("de-DE", "de-Latn-DE", "de", "en-US", "en-Latn-US", "en")

Windows display: de_DE + Apps & websites: en_US(:de_DE) + Regional format: de_DE -> both use de_DE

QLocale(): QLocale(German, Latin, Germany)
QLocale().uiLanguages(): QList("de-DE", "de-Latn-DE", "de", "en-US", "en-Latn-US", "en")

Windows display: de_DE + Apps & websites: en_US(:de_DE) + Regional format: en_US -> Kleo de_DE , GnuPG en_US
Windows display: de_DE + Apps & websites: de_DE(:en_US) + Regional format: en_US -> Kleo de_DE , GnuPG en_US

GetUserPreferredUILanguages: QList("de-DE", "en-US")
QLocale(): QLocale(English, Latin, United States)
QLocale().uiLanguages(): QList("de-DE", "de-Latn-DE", "de", "en-US", "en-Latn-US", "en")

Conclusions:

• GetUserPreferredUILanguages
  • always returns the Windows display language as first language regardless of Regional format and Preferred languages; this makes sense if one accepts that Preferred languages is not considered by GetUserPreferredUILanguages (which seems to agree with the API documentation)
  • returns only "en-US" if Windows display language is en-US, but returns "de-DE, en-US" if Windows display language is de-DE; this doesn't make sense to me, but that's Windows; the internet supports this observation: it seems that en-US is always returned as "fallback" if the Windows display language isn't already en-US
• Preferred languages (aka Apps & websites) seems to be completely ignored by Kleo and GnuPG; this isn't surprising since GetUserPreferredUILanguages also ignores it
• GnuPG seems to prefer the Regional format; this is because libgpg-error uses GetThreadLocale as mentioned above
• Kleo behaves strange:
  • If system lang and display lang are both de_DE then Regional format doesn't matter
  • If system lang and display lang are different then Regional format is preferred
• QLocale prefers Regional format (which makes sense since locale is more about numbers, dates, etc., than about languages)
• QLocale().uiLanguages() behaves as strange as Kleo; in fact, they behave exactly the same way, i.e. Kleo uses the first language in the list returned by QLocale().uiLanguages(); it's pretty obvious to me that QLocale().uiLanguages() is wrong/broken

Please create a separate ticket for "The initial language in kleopatra is not selected according to windows settings". This is a bug in KDE Frameworks that's orthogonal to the issues of this ticket.

Actually, Werner wants the possibility to change the language in Kleopatra (or Okular) to be removed from Gpg4win/VSD which would make this issue irrelevant for us.

Separate ticket for the "initial language selection": https://dev.gnupg.org/T7949