Hi,
MathGL have very limited set of built-in glyphs for ASCII, Greek and some mathematical symbols. This is subset of STIX2 font. So, basically you don't need any other font files. If you have a whole non-Chinese symbol set then just include STIX2 font (about 3 Mb) or any other (except largest ones, i.e. adventor, heros, bonum, chorus, schola, pagella, cursor, termes -- about 20 Mb). The largest font NotoSansCJK provide Chinese symbols.