So, we used to have a $messagecache->add(), but it was removed after a very very long deprecation.
Just when I was about to use it :) I basically look for this functionality:
* Figure out which localized string to use for language X, in case there is no message in X. Something like "use Russian if Belarusian message is not available". I remember this was present in pywiki framework, and suspect mediawiki has it somewhere.
* Expand {{templates}}/PLURAL/GENDER/...