fix: respect complex language strings when using validation

[michalsn]

Description This PR fixes a bug where complex language strings in the validation class are not handled properly.

The problem was that in the validation class, we were forming the final message using the str_replace() function, instead of using the built-in solutions that MessageFormatter::formatMessage gives us.

Checklist:

• [x] Securely signed commits
• [ ] Component(s) with PHPDoc blocks, only if necessary or adds value
• [x] Unit testing, with >80% coverage
• [ ] User guide updated
• [x] Conforms to style guide

[dgvirtual]

I tried it and it seems to work fine. Thanks @michalsn

[michalsn]

@dgvirtual Ok, thank you for testing it out.