STEPS TO REPRODUCE
1. Go to System Settings --> Personalization --> Regional Settings --> Language
2. Add on top of "American English (default)" the language "Nederlands". Put "Nederlands" at the bottom.
3. Save, reboot, run plasma/kde
4. On konsole prompt type an error (for example: sudo foobar). It will report
an error in Dutch. The direct reason for seeing Dutch is (apparently) due to
the environment variable LANGUAGE which is set to "en_US:nl". Changing that
into "nl:en_US" doesn't make a difference: still dutch error messages.
Only when changing LANGUAGE into "en_US" you get English error messages.
Everything is still 100% English (en_US), but now spelling checking also works when I type in Dutch.
There is another bug; if you choose in the list of Formats (not Language) en_NL - then everything breaks because that locale doesn't exist and is not supported. I think it SHOULD be supported: everyone in the Netherlands speaks English - this basically a two-language country, one of them being English. Aka en_NL makes a LOT of sense, exactly like all the other en_XY locales. Nevertheless, if I select it, then I expect it to work - and it doesn't: you can't generate that locale because certain idiots think it is not needed and refuse to add it (see for example https://sourceware.org/bugzilla/show_bug.cgi?id=14085)
This is Bug 192019. See the discussion there for the gory details. :)
TL;DR: if you want American English to be the system language, delete all other languages underneath it, as they will trigger this bug. If you want American English to be used as the *fallback* language, make sure it's positioned *last*, not *first.*
*** This bug has been marked as a duplicate of bug 192019 ***