This setting is for the language you are writing in NOT the onscreen language.
This changes the probability that your written language will be auto-translated. So if you write in Japanese, and set that in your Settings to Japanese, the software will automatically set language tags to Japanese for translation. Google will check prior to the tags being set to determine if it is more than 30% sure the written language you are writing in is wrong. If it is more than 30% sure, it will automatically change your language tags in a post to whatever it believes the language is.
The option in Settings does not affect your language as it appears on screen. This is done through the flags at the bottom of the screen. We have disabled the browser check for your language after finding bugs with Chrome. The default language when you come to this site will always be English, unless you change it using the flags at the bottom of the page.