Help text lang strings do not cache

During testing of MDL-62439, I noticed that when in clean, an ajax request is sent everytime you click on the help icon. Even though there is a check on a 'textcache', it doesn't seem to be doing the caching as expected. Found when editing course settings.

Steps to reproduce it: 

1. Switch to clean theme
2. Go to a course and edit it's settings
3. Click on the  icon
4. Notice an ajax request in inspector every time you click it.