There is plenty of evidence that localization of your mobile application increases sales and distribution. It
Unless you are a native speaker of several different languages, the best way to get your app translated is to outsource it to a company that specializes in this type of translation. There is a good list of companies on Apple’s internationalization page, but several developers we like to recommend Tethras.
Your application should react to the locale (local codes) and not just how text is displayed (left <-> right) or date formats (US <-> EU) but also in the assets like images and audio, particularly when the content goes beyond a beep and star which they are internationally understood.
How many languages?
The languages into which you choose to translate your app really depend on which markets you are trying to hit. We recommend if you are trying to reach a broad market, you should strive to hit at least English, Spanish, German, French and Chinese.
I make heavy use of iTunes Connect and more than often I know the Apple documentation by the heart. Most recently I happen to notice that the list of supported languages was missing from their site. They have been expanding a lot so it is possible that the doc team can’t keep up.
Apple is by far one of the most responsive makers and developers when it comes to internationalization. Their SDKs supports more languages than the documentation says, I can tell you that :-)
I built a little app (Github) that extracts directly from the babe’s mouth (the SDK) all the supported languages. The app is very simple and it has the benefit that it will stay ahead of any documentation for sure as it relies directly on what the SDK supports.
Don’t assume that each [..]OS supports all the languages, for example, tvOS has most recently added additional support previously wasn’t available.
All the above also applies to Siri's capabilities of understanding and responding in the target language. Something to keep in mind if you make any use or leverage on my most favorite and sexy voice assistant. I found Wikipedia to do a very good job of keeping track of Siri polyglot skills.
If you want an end to end class of making your application fully localized take a look at this post by Ali. It is for Objective-C however it should very little trouble to translate in Swift 2.x and if you really hit one or two roadblocks in getting the syntax right, give a shot to this tool. It translates Obj-C in Swift and for the few instances that I have used it, I did the job right.
There’s another localization place that it also not documented and that it is the languages supported by the iTunes Store. That is the number of languages you can write your metadata before submitting the app for review. At the time of writing this, it supports 28 languages. This link claims that they keep the list up to date. Their current list matches mine as of March 2016.
keep your users happy. Localize, do it right, do it for as many as you can. As a multilingual, I am definitely prone to buy your app if I see that you support more than just Spanglish and other macaronic languages.