What we would really need is a modded app, where no matter what the language in your system is set to it would only apply English language to the Mi band. That would do the trick.
i think that by doing that you should flash back the original firmware, only then it´ll all be in english, the problem here is the custom fw is missing some translations/strings i think.
Have you tried using notify & fitness for Mi Band app from play store? It has a function to install custom fw and it works perfect. Don't even need to activate the detection feature.