Edit: Nope, it's XSalsa20.
libsodium adds ChaCha20 (https://download.libsodium.org/doc/advanced/chacha20.html) but not XChaCha20.
why can't we get our shit together...
aes-128 in counter mode, Salsa20 with 8, 12 and 20 rounds and XSalsa with 20 rounds.
Som other implementations of NaCl/Sodium have restricted the different algs. TweekNaCl only supports XSalsa20 and Salsa20.