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/Providers/TranslationServiceProvider.php

@@ -3,10 +3,41 @@
 namespace BookStack\Providers;
 
 use BookStack\Translation\FileLoader;
+use BookStack\Translation\MessageSelector;
 use Illuminate\Translation\TranslationServiceProvider as BaseProvider;
+use Illuminate\Translation\Translator;
 
 class TranslationServiceProvider extends BaseProvider
 {
+    /**
+     * Register the service provider.
+     *
+     * @return void
+     */
+    public function register()
+    {
+        $this->registerLoader();
+
+        // This is a tweak upon Laravel's based translation service registration to allow
+        // usage of a custom MessageSelector class
+        $this->app->singleton('translator', function ($app) {
+            $loader = $app['translation.loader'];
+
+            // When registering the translator component, we'll need to set the default
+            // locale as well as the fallback locale. So, we'll grab the application
+            // configuration so we can easily get both of these values from there.
+            $locale = $app['config']['app.locale'];
+
+            $trans = new Translator($loader, $locale);
+            $trans->setFallback($app['config']['app.fallback_locale']);
+            $trans->setSelector(new MessageSelector());
+
+            return $trans;
+        });
+    }
+
+
+
     /**
      * Register the translation line loader.
      * Overrides the default register action from Laravel so a custom loader can be used.

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);
+    }
+}