My fiancee is Vietnamese. Her English is poor and my Vietnamese is also poor yet we manage to have 1.5 hour telephone calls. I personally do a lot of thumbing through my Vietnamese-English dictionary when I'm talking to her. Our conversations are not very complex but we are communicating. I talked to a USCIS immigration officer about her poor english skills and he told me that it would have no bearing on whether she was granted a K1 Visa because she could request a translator. I understand that the consulate officer may bring up the question about how we communicate. We talk to each other in a English/Vietnamese mix. I included a screenshot of such a conversation via Yahoo IM with my petition. She continues to study English and I continue to study Vietnamese. Should I worry?

All I can tell you is that I don't think her not knowing English will be a problem at the interview. My fiance does not speak English, but our case is different because I'm fluent in Portuguese.
Maybe someone else can help you out....?

English is in no way a requirement to get a visa, but at the interview it's rather likely they will question how you communicate when neither of you are fluent. No need to worry, just be prepared to explain just as you did here.

I hate to say this, but yes you should worry a little bit. The HCMC consulate will want to know how you communicate and if it comes to question they will require hard evidence that you can. This usually comes in the form of a video tape showing the two of you carrying on a conversation together in person- in either language.

Thanks for the input. I am worried now a little bit but at least maybe I can be more prepared. I just mailed the I-129F yesterday so we still have at least 7 months to prepare for interview.