How about them locales?

Earlier, I encountered a strange error message, or actually several.

The message was that there was something wrong with my locales, and the system was falling back to a default named ‘C’.
When I tried to dpkg-reconfigure the locales, I got a message saying that the LC_default was not set, as was another one. The others were set to Dutch locales (nl_something UTF8).
If I remember correctly I had my language set to UK English, my formats to Dutch, and my input source to Dutch as well.

Guessing this was were the problems lay, I changed the language and the input source to US English.

Now the errors seem to have gone. Or so I hope.

This begs the question: do these problems arise when the language and/or the input source are set to something else than US English? Or could this be down to something going wrong during the installation or removal of an app, or something like that?