Apple's documentation indicates that Big Sur should have at least the following new language features:
+New bilingual dictionaries include French–German, Indonesian–English,
Japanese–Simplified Chinese, and Polish‑English.
+New fonts for India include 20
new document fonts. In addition, 18 existing fonts have been upgraded with more
weights and italics
+Enhanced predictive input for Chinese and Japanese results
in more accurate and contextual predictions
I have found 4 additional items on the spellcheck language list compared to Catalina: Irish, Romanian, Punjabi, and Telugu.
Big Sur also lets you switch keyboards via the Fn key, which has a globe printed on it on the latest hardware.