Twitter update adds Tweet translation to help you read foreign language messages
Even though Bing Translator uses advanced language software, in some situations a translated tweet will not be accurate, which is why the original text can always be found above the translation. If you'd rather not have a foreign language tweet translated, you can go into the settings and turn the feature off. At first, Tweet translations will support over 40 language pairs.
Prior to using Bing for its new feature, Twitter relied on volunteers who would translate foreign language tweets by hand.
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source: @twitter via AndroidCentral