There's some very useful information there and you should definitely use the language tag when you can. However, search engines do not rely on it much for one very simple reason: so few people use it it's not very accurate. The vast majority of sites have their language tag set to 'en' - even when the language is not English. I once did a test of European Government sites and virtually all had the 'en' tag. If you're a search engine, that means it's not a very good signal and you have to rely on something else. That something else is most likely to be a local domain, clear un-mixed language content and inbound links from sites in the targeted language.