think there's been a good English translation for "tsundere"
I think that English should just adopt the word.
https://en.wiktionary.org/wiki/tsundere already lists tsundere as loanword so we're part way there. I agree that there is no adequate equivalen word in English that has a substantially similar meaning - you basically have to provide a definition of the word to properly convey it (e.g. saying someone "runs hot and cold" or "they act haughty but they also have a bashful side")