So flags aren’t languages, but they do give a good signpost to where the language selection is on the page.
Take for example Mozilla’s add-ons website. I somehow ended up on this language version of their website:
Knowing it wasn’t the right language for me I began hunting for the option to change my language, which was eventually found at the bottom of the page:
It might have been because I count myself as a “smart web user” that I looked in the dropdown list to see if English was written there. Or it might have been because it was the only place on the page where it looked like there was a hidden element, but that’s how I ended up being able to switch the language on the Mozilla site back to English.
User testing
I ran a small user test to see what others would do, the participants scrolled up and down the page, and stated that they could not see anything that indicated to them that there was a way of changing the language, and gave up. One then tried to go to Google translate to translate the page.
I also tested the same participants on another site that used a globe icon as a symbol for language selection, but that was also missed as a language selection cue. My opinion is that a more noticable icon, e.g. bigger or more lifelike (colour) may result in this language selection cue producing better results if I had run a follow up test with other participants.
Conclusion
Flags aren’t languages but they are definately a language indicator which users would use to find a way of changing the language of a web page.
Using the name of the language translated directly on the page and not as a drop down wouldn’t work on this site because its simply too cluttered, thus reducing the findability. The footer would need to be rearranged with more white space to make the area more noticable.
Here is the page by the way, so you can have a look for yourself: addons.mozilla.org/zh-CN/firefox/