From my own family experience:
Look, I tried teaching, coaching, pulling my brothers and sister into English while I was still at home. I tried drilling into their brains that this was useful stuff. Did they listen? No. So what happened? After I left and they realized they needed English as a working tool, they all had to start the hard road without any help. No more big sister to help them with their homework. Currently, only the smallest is fluent, and God knows he was the one I was the hardest with, I feel guilty, but it worked.
So, until your wife really needs it, she won't be motivated enough.If she does not have the desire, the need, it is easier to keep a basic level. As they have said, talk to her in English -it is better for the kids, too, one parent talking in English, one in Mandarin.
I coached my father when he went for his PhD and had to take exams in English. Afterwards, he continued on and off on his own, and now he can hold a basic conversation, which is quite useful since he lives in a touristic town and now he has to ferry my US uncles around when they visit by himself, no more extra help from me. These interactions helped him reach a better level. maybe when family comes over you can also delegate responsibilities to your wife, she can see by herself how and where her extra language skills can become useful.
With my mom I gave up, she always relied on me as translator since I was 6, and simply did not want to start, doesn't want to start, same as with driving since when I left, she ended without her chauffeur -me- but with a perfectly running car in the garage. Same with computers. Her case is an example of "you can take the horse to water" as she's the one who has traveled more times abroad of all the family and stayed the longest in the US yet simply does not want to learn. She can go shopping by herself, and that is all she cares about, then grows bored as she cannot even watch TV.
But it's her choice. So respect your wife if that is her choice. many people here live in Taiwan and do not learn the language. How useful or interesting your wife finds English depends on her. maybe you can entice her with some magazines or TV program she likes. that is all I can think of.
Finally, some people say you should not teach family, like back home they say a husband should not try teaching a wife how to drive. maybe she will listen to an outsider more. remarkably, the best English teacher my brother had was a coworker of mine, whose level of English, honestly speaking, is lower than mine, but she is an excellent educator, can explain things better in a more entertaining way that I could ever do. I helped her land that job and I am now glad I did. My brother listened to her more than he would ever listen to me. And took homework more seriously. So maybe you can talk your wife into some tutorial, with someone you know?