google has Mongolian and Khmer fonts under open font license. For some reason they prefer not to design their site to work on system which has Mongolian and Khmer support, if javascript is not enabled.
Old Italic and Linear B support in subtitles was a joke.
It might be better, if UI did not depend on single font for subtitles and could use any font that supports text in specific unicode block.
Noto Sans Mongolian - Google Fonts
Noto is a global font collection for writing in all modern and ancient languages. Noto Sans Mongolian is an unmodulated (“sans serif”) design for texts in the C
fonts.google.com
Khmer - Google Fonts
The Khmer fonts are designed for readable and beautiful rendering of text in the Khmer script, the national language of Cambodia. The designer, Danh Hong, has m
fonts.google.com