Useful links to tex.stackexchange

• A very simple example with Russian and German:

https://tex.stackexchange.com/questions/519267/how-to-combine-russian-and-german-language/519270#519270

• An example showing how to organise the code with many languages:

https://tex.stackexchange.com/questions/513618/howto-multilanguage-babel#513667

• How to write Korean words in an English text, with automatic font switching:

https://tex.stackexchange.com/questions/523610/korean-symbols-in-latex#524636

• An example with English (australian) as main language, simplified and traditional Chinese, and Pinyin:

https://tex.stackexchange.com/questions/499910/mixing-chinese-characters-with-english-main-language-in-2019-what-is-the-st#499992

• Another example, this time with Hebrew, traditional Chinese, and simplified Chinese:

https://tex.stackexchange.com/questions/483605/creating-a-document-with-mixed-languages#483612

• With German, Korean, Arabic and Chinese:

https://tex.stackexchange.com/questions/492198/how-to-write-korean-and-chinese-within-an-german-document-with-other-european-la#492214

• An explanation of the warnings with \babelfont:

https://tex.stackexchange.com/questions/511079/why-babel-produce-warnings-about-rmfamily-and-ttfamily-with-babelfont#511296

• Mixing Japanese, Korean, and tradicional Chinese:

https://tex.stackexchange.com/questions/480132/multiple-xecjk-fonts#480155

• Different font features for different Greek variants:

https://tex.stackexchange.com/questions/542882/tags-for-polytonic-greek-with-babel/543023#543023

• Sanskrit, somewhat old but still useful, I think:

https://tex.stackexchange.com/questions/407407/fonts-are-not-applied-to-text-in-lualatex#407461