I just tried LokeshHK's suggestion and that seemed to work for me. However, I had to clear my browser cache and reload the page in order for the language changes to fully stick.
Are you certain that you tried it with the HTML5 client? Can you specify which version, browser and operating system and which languages you tested?
I could not get it to work so far in both Vivaldi and Safari using private sessions (on MacOS 10.12.6)
Currently this makes me not want to use the HTML5 client even though it is vastly better performing than the flash version, because the german localisation uses very different terms from the english one and the latter is simply more consistent and more useful for research and documentation.
In vSphere 6.7 you can simply click your login name in the upper right corner of thje screen, select "My Preferences" > "Language" and switch between "Browser default" and "Specify a language", where the latter enables you to choose between English, German, Spanish, French, Korean and three other Asian languages which I'm not able to identify