I noticed another things. The feature seems only to work when google search is a system app. It comes as stock in my current firmware (BR stock 4.4.4) and I usually uninstall all the google clutter from system (and replace some of them with user installation) but with google search I took the care of backing it up. When restored the "ok google everywhere" feature could not be activated anymore even if restored to system (and pretty much messes everything). If deleted and reinstalled from playstore you never get the feature again even if you push the app to be a system app.
Also it works for me to use dual languages, leaving US English as default I don't lose the "everywhere" feature.