Localize validation messages with user lang files

[kjac]

Prerequisites

• I have added steps to test this contribution in the description below

Description

Although currently undocumented, it is possible to use user lang files to localize the backoffice (instead of using dictionary items). However, that kind of localization does not extend to property validation messages.

This PR fixes it.

Testing this PR

Create a user language file like this in [site root]\config\lang:

<?xml version="1.0" encoding="utf-8" standalone="yes"?>
<language>
    <area alias="validation">
        <key alias="mandatory-field">It's mandatory</key>
        <key alias="numeric-field">It's got to be a number</key>
    </area>
</language>

Use these language keys to configure property validation messages:

Now verify that the validation messages are indeed utilized by the backoffice:

...also in block based editors:

[Zeegaan]

LGTM 🚀