tl;dr
Usually you should be safe by offering your Plugins in English first. If you create a good userbase, you could do your own resarch as to find out which languages are most important to your clients.
Another thing to consider would be, if you want to support latin-signs only or if you want to offer all kinds of language signs.
The big prolbem (for me) is, that I have absolutely no idea if something is correct in another language, where I cannot even read the letters.
My unofficial list
After doing a little resarch (to be found below), I would suggest the following priority. My list is based on personal experience, research and common sense (languages a lot of people learn are more commonly supported)
- English (first, nearly every programmer has to have at least basic knowledge of it)
- Spanish
- German
- French
- Italian
- Russian
- Japanese
- Chinese
- Portuguese
- Turkish
- Arabic
- Korean
Sources
some statistics of wordpress.com.
I know, it is wordpress**.com**, but it is the first step to some overview for your most important languages.
w3tech has a statistic for content by language
Wikipedia‘s statistics are quite the same:
However, if you take into account the number of internetusers speaking a specific language, it gets a little different – internetworldstats.com.
