I’ve just tried the Polyglot plugin for Chrome (there’s also a Firefox equivalent) with Mandarin Chinese. The idea behind the plugin is to provide a bit of daily reading and vocabulary practice in your target language by randomly replacing a few words on web pages with translations.
On the whole it works fairly well, and you can adjust preferences for the proportion of words that will be translated, the target language and so on.
It’s a good feeling to read the news or a blog and get a little bit of language practice at the same time. Whether this kind of passive learning actually works is hugely controversial, but at the least it seems safe to assume that it doesn’t hurt. Except of course, if you’re passively learning complete rubbish.
Within the first five minutes of using the plugin, some pretty ridiculous translation errors came up. One was the phrase “safe and well” being converted to “safe 和井” - that is, translating “well” into the thing you get water out of, not the state of being OK .This is a classic, schoolchild translation mistake.
Because of this, anyone using Polyglot needs to be pretty vigilant about what it’s doing, and certainly shouldn’t be noting down it’s creations for further study. However, for learners of Mandarin and Japanese, the plugin isn’t much good anyway unless you’ve already got a fair bit of learning under your belt - you just won’t be able to read the characters.
The plugin certainly isn’t going to teach you grammar or improve your fluency, but it does make you feel less guilty about spending time on unproductive tasks like blogs. For that reason, I’ll carry on using it.