Change Default System Font?

Hello,

I've been wanting to reverse a "feature" I requested for LibreOffice for
two years now but haven't been able to get any traction (the feature
request and then rescinded request is here:
https://bugs.documentfoundation.org/show_bug.cgi?id=52020 ). So I am
preparing to do a build of LibreOffice with the feature removed myself so
that Khmer and Minority Language users affected can go back to using
LibreOffice again.

But since I haven't used LibreOffice for the Khmer language for sometime I
recently downloaded LibreOffice 5 and found I was unable to change the
menu/system font to a font that supported Khmer.
Previously I was able to do this by Tools > Options... > LibreOffice > View
> User Interface section > uncheck Use system font for user interface and
then use a font replacement table for Andale Sans UI and replace it with
whatever font I want.

Because I can't change the system/menu in LibreOffice to a font that
supports Khmer, even if I am successful at my own build, it would be
worthless since we use spelling and grammar checkers that need Khmer in the
system/menu font to be usable.

Can anyone help me out by directing me to the source code that would need
to be changed in order to specify my own system font for LibreOffice?

Thanks for your time and consideration,
Nathan Wells
sbbic.org