I can use e.g. Awesome skin or one of the other glyph themes with the rtl: true option added.
This makes it rtl, but one detail does not work. The expander points still to the right instead of to the left, if a tree is not expanded.
My impression is that there is no mapping for rtl version of this symbol? Did I miss it or is there already a manner to flip this symbol? Alternative is to remap it, but I like to switch on the fly between languages, so that means more changes are needed per language.
Also no clues on https://github.com/mar10/fancytree/wiki/ExtGlyph for rtl.