The trouble with using partial translations, for non-English speaking users (or really any language) is that as soon as they hit a page that's not in their language...they think "now what?" That's an obvious problem. There are ways to update your site content for various languages relatively cheaply too. However, there are alternatives.
If you do have language variants for certain pages, and you are using cookies or logins to determine whether that user is English or another language, then it would be relatively painless to integrate Google Translate so that those pages automatically translate to the described language. Then, for the sake of not clearly exploiting Google Translate instead of accommodating your viewers, I'd suggest including a basic text prompt at the top of the page that says something to the effect of "we're working on translating the entire site, mind our use of Google Translate".
In short, it's a very bad practice to have only select content work correctly on a website because it causes confusion and is annoying to users of that language. Best practice is to not do it at all.