Where does this information come from? It is never stated as such in R2 so, what side information conveys this?
Empress Tianzi's real name is never stated in the anime series; it's only stated in the Code Geass: Lelouch of the Rebellion R2 official guidebook.
According to pixiv encyclopedia (Japanese),
[...] Tianzi's real name. Even though her real name does not appear in the TV anime, it was stated in Code Geass's appendix booklet that was released before the last episode. [...]
Researching further, based from tare-pannda's blog post (Japanese), her real name was stated in Code Geass Lelouch of the Rebellion R2 The Complete Official Guide Book (Japanese) where the blogger mentioned the content of the book:
DRAMATIS PERSONAE ～キャラクター解説～
DRAMATIS PERSONAE ~Character Explanation~
[...]・Tianzi (Jiang Lihua) [...]