Why would you need to learn another language?
Well if you�re not leaving your own country or partaking in any of your countries other cultures, then there is no reason whatsoever.
However if you are going to a foreign country or wish to partake in your countries other cultures, then for no other reason then common courtesy.
Top 30 most spoken languages
http://www.krysstal.com/spoken.html
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English Riviera:
Live entertainment, Real Ale, Great Scenery, Great Beaches, why would anyone want to live anywhere else (I�m definitely staying put).