You are correct that the user error texts in the System module are only available in US English, not UK English. Currently, there is no way of adding these texts to the System module in other languages.
What you could do, however, is to add the US English language to your project and make it the default language. Making it the default language means that it is the fallback language that is used when texts are not available in the current language.
Your users can still use UK English as their language. However, due to US English being the default language, the user error texts are displayed correctly as well.
The other problems related to languages and texts can be solved using the Language Operations dialog that was explained in a recent tech-tip. For example, if you downloaded an app that has texts in US English but not UK English, you can use that dialog to copy those texts to UK English.
Do you know if those system.user error message texts can be modified if using English-US as default language? LR.