diff --git a/lib/moodlelib.php b/lib/moodlelib.php
index 43c3e57e5e7..46e2ebc3beb 100644
--- a/lib/moodlelib.php
+++ b/lib/moodlelib.php
@@ -6293,7 +6293,7 @@ class core_string_manager implements string_manager {
             $component = $plugintype . '_' . $pluginname;
         }
 
-        if (!$disablecache) {
+        if (!$disablecache and !$disablelocal) {
             // try in-memory cache first
             if (isset($this->cache[$lang][$component])) {
                 $this->countmemcache++;
@@ -6384,12 +6384,14 @@ class core_string_manager implements string_manager {
         // we do not want any extra strings from other languages - everything must be in en lang pack
         $string = array_intersect_key($string, $originalkeys);
 
-        // now we have a list of strings from all possible sources. put it into both in-memory and on-disk
-        // caches so we do not need to do all this merging and dependencies resolving again
-        $this->cache[$lang][$component] = $string;
-        if ($this->usediskcache) {
-            check_dir_exists("$this->cacheroot/$lang");
-            file_put_contents("$this->cacheroot/$lang/$component.php", "<?php \$this->cache['$lang']['$component'] = ".var_export($string, true).";");
+        if (!$disablelocal) {
+            // now we have a list of strings from all possible sources. put it into both in-memory and on-disk
+            // caches so we do not need to do all this merging and dependencies resolving again
+            $this->cache[$lang][$component] = $string;
+            if ($this->usediskcache) {
+                check_dir_exists("$this->cacheroot/$lang");
+                file_put_contents("$this->cacheroot/$lang/$component.php", "<?php \$this->cache['$lang']['$component'] = ".var_export($string, true).";");
+            }
         }
         return $string;
     }
@@ -6469,8 +6471,11 @@ class core_string_manager implements string_manager {
                 return 'en';
             }
             if ($this->usediskcache) {
-                // maybe the on-disk cache is dirty - let the last attempt be to find the string in original sources
+                // maybe the on-disk cache is dirty - let the last attempt be to find the string in original sources,
+                // do NOT write the results to disk cache because it may end up in race conditions see MDL-31904
+                $this->usediskcache = false;
                 $string = $this->load_component_strings($component, $lang, true);
+                $this->usediskcache = true;
             }
             if (!isset($string[$identifier])) {
                 // the string is still missing - should be fixed by developer