Fixed incorrect pluralisation for de_informal

Updated language system to only use initial part of locale for
translation pluralisation to better match the hard-coded logic of the
built-in MessageSelector. Extends and overrides Laravel's default for
this system.

Added test to cover.
Related to #3976.

app/Translation/MessageSelector.php

@@ -0,0 +1,19 @@
+<?php
+
+namespace BookStack\Translation;
+
+use Illuminate\Translation\MessageSelector as BaseClass;
+
+/**
+ * This is a customization of the default Laravel MessageSelector class to tweak pluralization,
+ * so that is uses just the first part of the locale string to provide support with
+ * non-standard locales such as "de_informal".
+ */
+class MessageSelector extends BaseClass
+{
+    public function getPluralIndex($locale, $number)
+    {
+        $locale = explode('_', $locale)[0];
+        return parent::getPluralIndex($locale, $number);
+    }
+}