Is it safe to delete large OS X font files?
Deleting any fonts in /System will require the disablement of System Integrity Protection (&quot;rootless feature&quot;) on OS X El Capitan. SIP reduces the potential attack surface for system exploits; disablement is not recommended.
The recommended method of deletion for any other fonts is through Font Book (rather than a direct deletion using Finder, Terminal or a cleanup application).
This question and accepted answer were originally linked to a Lion issue, but the basis should still apply:

You can save several dozen megabytes by deleting certain Asian fonts if you don't need them. Don't delete system fonts directly in the Finder.
Rather, do it through the Apple Font Book application, which will prevent you from deleting the &quot;reserved&quot; system fonts that Mac OS X expects to see when it boots up, but permit you to delete &quot;non-essential&quot; fonts.

If OS X is unable to render the character set with the required font, it is likely that a &quot;substitute symbol character&quot; will appear instead.