Ai Uehara used to sometimes respond to english tweets from fans with stuff like "Thank You". The only catch, she wrote it all in katakana which I always found funny, because I was never sure if she did it in a jokey way, or if she geniuinly didn't know that most foreigners probably can't read kana, even if she technically wrote english.
I've seen some english back and forth tweeting on Yuu Kawakami's account, well, she tried to, at least, though not with a fan but rather one of the american actors she played with in one of her interracial flicks.
https://twitter.com/yuu_kawakami/status/330218458388234241
There was a bit more, but hell if I can find it again. It doesn't help that I barely get how Twitter works.
Maki Hojo sometimes responds to english tweets.
Then there's Ruka Kanae who used to have her own english twitter account.
https://twitter.com/jpavidolruka
She appearently stopped using it last november and she rarely responded to other tweets, I think she just used it as a simple english practice more than anything else, but it's something at least.