[Feature] 11253 copy project error messages always in english

[``* `#11253` Copy project always returns non-localized error messages``](https://www.openproject.org/work_packages/11253)
I've also had a similar dilemma here: https://github.com/opf/openproject/pull/1550/files#diff-127a850b7d7b20173ad3fba88f8a25e3R35 (feedback welcome btw)

I think Helper-style modules, that do not have an associated Controller, probably belong in `lib`.