Localization will be worth it if:
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You're targetting casual gamers - Casual gamers are... casual. Their English proficiency is usually much lower than hardcore gamers. Your store description alone will filter them out if it's in English only, so localization matters a lot for them.
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Understanding the game is essential - If you have to explain gameplay mechanics or if you're telling a story, consider localization or people might get frustrated and leave bad reviews because they can't play the game or fully understand it.
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There's a potential audience for your game in foreign markets - Do some market research to see if similar games do well in certain markets, if there's a lot of competition, if you can have visibility, etc.
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Your game already has some visibility - If it doesn't, localization will still help it reach more people, but it won't do miracles either and you may not get ROI.
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It's well executed - This point is especially key if your game is narrative-heavy. Sloppy translations ruin the experience, so make sure you get quality translations.
If you're interested in reading stories on that topic, we recently asked 3 indie studios if localizing their game was worth it:
http://www.leveluptranslation.com/single-post/is-it-worth-localizing-an-indie-game