There is no provision for determining if a font can support a language.

This was found when investigating RT-36574, (Arabic letters in Ensemble8 not displaying).
The system font did not have Arabic (ar) support, and so the result did not look proper.

But... unlike AWT.font which has methods like canDisplay(int codePoint) and canDisplayUpTo(String str), there is no programatic way to determine what a given font support. Interestingly for me - there appears to be no way to ask for a font that does support a code page in AWT, so it looks like you need to know what your system supports.