MDL-64757 core_output: fixed incorrect string language mustache blocks

Mustache blocks nested in parent templates were incorrectly rendering the default language Moodle.
This fix correctly passes the current language preference to all nested mustache templates such as blocks
which are children of another template.

lib/classes/output/mustache_template_source_loader.php

@@ -158,7 +158,7 @@ public function load_with_dependencies(
         // Get the requested template source.
         $templatesource = $this->load($templatecomponent, $templatename, $themename, $includecomments);
         // This is a helper function to save a value in one of the result arrays (either $templates or $strings).
-        $save = function(array $results, array $seenlist, string $component, string $id, $value) {
+        $save = function(array $results, array $seenlist, string $component, string $id, $value) use ($lang) {
             if (!isset($results[$component])) {
                 // If the results list doesn't already contain this component then initialise it.
                 $results[$component] = [];
@@ -173,7 +173,7 @@ public function load_with_dependencies(
         };
         // This is a helper function for processing a dependency. Does stuff like ignore duplicate processing,
         // common result formatting etc.
-        $handler = function(array $dependency, array $ignorelist, callable $processcallback) {
+        $handler = function(array $dependency, array $ignorelist, callable $processcallback) use ($lang) {
             foreach ($dependency as $component => $ids) {
                 foreach ($ids as $id) {
                     $dependencyid = "$component/$id";
@@ -223,7 +223,8 @@ function($component, $id) use (
                 &$seenstrings,
                 &$templates,
                 &$strings,
-                $save
+                $save,
+                $lang
             ) {
                 // We haven't seen this template yet so load it and it's dependencies.
                 $subdependencies = $this->load_with_dependencies(
@@ -232,7 +233,8 @@ function($component, $id) use (
                     $themename,
                     $includecomments,
                     $seentemplates,
-                    $seenstrings
+                    $seenstrings,
+                    $lang
                 );
 
                 foreach ($subdependencies['templates'] as $component => $ids) {