Inherted lang attribute should be below the default in the chain

It was pointed out in #5468 that the `lang` attribute, when inherited
from a parent element, was above the option set in the global default
within the inheritance chain. While this makes sense because of how
we inherit other properties, it does not make sense for the `lang`
attribute.

The inheritance chain for the `language` option has been adjusted
to be the following:

1. The `lang` attribute on the original `<select>` element
2. The `data-language` attribute on the original `<select>` element
   (because of how `data-*` attribute resolution affects options)
3. The `language` option specified when initiailizing Select2
4. The `language` Select2 default
5. The `lang` attribute on a parent of the `<select>` element

While this is a breaking change, we believe that this change will
have minimal to no impact on real-world usage of Select2, because
of how the `lang` attribute is generally used within documents.
We believe this will now make setting the default language through
JavaScript easier and more reliable, bringing it in line with how
we recommend it is done within the documentation.

This was implemented through a new method `Defaults.applyFromElement`
instead of within the old `Options.fromElement` method because it
relies on the global defaults object. While we could have reached
in to the internals in order to apply it appropriately, it made
more sense to handle the proper resolution through a single
consistent place.

This was not implemented in the `Defaults.apply` method because that
method is not typically passed in a reference to the original
`<select>` element that it is applying the options for.

Closes #5468

src/js/select2/defaults.js

@@ -232,8 +232,6 @@ define([
       );
     }
 
-    options.language = this._resolveLanguage(options.language);
-
     // Always fall back to English since it will always be complete
     options.language.push('en');
 
@@ -341,6 +339,24 @@ define([
     };
   };
 
+  Defaults.prototype.applyFromElement = function (options, $element) {
+    var optionLanguage = options.language;
+    var defaultLanguage = this.defaults.language;
+    var elementLanguage = $element.prop('lang');
+    var parentLanguage = $element.closest('[lang]').prop('lang');
+
+    var languages = Array.prototype.concat.call(
+      this._resolveLanguage(elementLanguage),
+      this._resolveLanguage(optionLanguage),
+      this._resolveLanguage(defaultLanguage),
+      this._resolveLanguage(parentLanguage)
+    );
+
+    options.language = languages;
+
+    return options;
+  };
+
   Defaults.prototype._resolveLanguage = function (language) {
     if (!language) {
       return [];

src/js/select2/options.js

@@ -11,6 +11,10 @@ define([
       this.fromElement($element);
     }
 
+    if ($element != null) {
+      this.options = Defaults.applyFromElement(this.options, $element);
+    }
+
     this.options = Defaults.apply(this.options);
 
     if ($element && $element.is('input')) {
@@ -34,14 +38,6 @@ define([
       this.options.disabled = $e.prop('disabled');
     }
 
-    if (this.options.language == null) {
-      if ($e.prop('lang')) {
-        this.options.language = $e.prop('lang').toLowerCase();
-      } else if ($e.closest('[lang]').prop('lang')) {
-        this.options.language = $e.closest('[lang]').prop('lang');
-      }
-    }
-
     if (this.options.dir == null) {
       if ($e.prop('dir')) {
         this.options.dir = $e.prop('dir');