I have noticed that it is common on StackOverflow for questions to use "on ⟨programming language⟩" where American English would require "in". For example, "Is there a getInt function on Haskell". Is there a particular English dialect where this usage is common, or is this more likely a mistake by a non-native speaker?
First of all, a short answer is just that I, as a Brit, have never heard anyone say on [language] and I, as a programmer, have always said in [language]. Indeed a cursory search of Google books would suggest that the phrasing program on (presumably followed by the name of a language in this case) is mainly used in AmE to say something like
This is a programme on compound semiconductors.
More in the sense of a research trial or as the OED puts it:
A set of related measures or activities with a particular long-term aim: the British nuclear power programme
While on the other hand program in comes up with lots of references to computer programming, for example:
How Not to Program In C++
(The top search result)
Furthermore, a Google search specifically of Stack Overflow shows that the phrasing program on is more often used in reference to operating systems or devices and not individual programs or languages.
Finally, if we look at the phrase program in from Stack Overflow it comes up with lots of references to
I program in [given language]
I would therefore suggest that you always use in as the preposition here if you're talking about a language.
The person who said "on Haskell" is someone called Mario from the Universidad de Granada. The chances are that he is not a native English speaker. He is probably Spanish. It's very easy to get prepositions wrong in foreign languages. Mario simply got it wrong. I've been a professional software developer for many years, and I've never heard anyone say "on [programming language]".