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:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• 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.