[jamesoneill]

For a while I had my system language preferences set as en-UK, ja, en-US and every so often I'd get an installation that was in Japanese because there was no UK English version.

[JimDabell]

The country code for the United Kingdom is GB, not UK. There's no such language code as en-UK. British English is designated en-GB.

[LeoPanthera]

That sounds correct behavior to me. You should add a plain "en" before ja.

[PhasmaFelis]

Is that a problem? It sounds like what you asked for.