@stefan.chirila @Morgan_Hardwood I have updated all the AppImage packages in Release Automated Builds · Beep6581/RawTherapee · GitHub with the fix to the fonts issue.
Finally I went for a more complicated, but better, solution to this problem. The version of the fontconfig library that is used is determined at runtime, based on the versions of the bundled and system libraries. If the system library is newer than the bundled one it will be used, otherwise the bundled one takes precedence.
Thais seems to be working fine on all systems I tested, ranging from Ubuntu 14.04 to Debian-testing.
Could you please double-check it and, if ok, update the official link in the RT Downloads page?
Thanks!