I learned english by playing Magic the Gathering! Well not solely of course, I had english classes, english movies, english books and a lot of other stuff in english, but I will make the bold statement that Magic the gathering is the reason I went in to all of those other things with the enthusiasm I did.
When Magic the Gathering was launched in 1993 I was 11 years old and my english was pretty much limited to "my name is" and "hello".
Then my cousin introduced me to Magic the Gathering and I had to learn english. My cousin is a couple of years older than me and in 1993 his english wasn't really stellar either, but together we spelled our way through the rule book and the card abilities and flavour texts.
I will admit that for the first couple of years we were playing, we probably made up a lot of rules, we played Magic the Gathering as we thought it was meant to be played based on our limited english skills. If something was too hard for us to translate we would just ignore that part. This was true for Cumulative upkeep for instance. (Those cards were ridicolously overpowered in our games until we grasped the meaning.)
From I was 11 and until I was 20 my grandmother would give both me and my two cousins booster packs for christmas and we would spend the holidays together playing and constructing decks. I don't buy as many cards anymore, however I still love to play Magic the Gathering and to see all the new abilities and additional rules Wizard of the Coast adds every year to keep the game vibrant and interesting. If you have a child age 11 or up, this may be the perfect gift for them, both in terms of enjoyment and socialising, but also as a way to make them want to learn english. (this is helpful for both non native speakers and for native speakers as their vocabulary is bound to be expanded either way.)