This is perhaps a honey-trap for the suitable-question-enforcers, but it came up in conversation with some other developers, and it's certainly related to programming languages. And I don't think it's argumentative, which is the other common justification for closing a question. We'll see ...
Try checking Wikipedia and Reddit out. The first one is a list of all programming languages, and the second is a discussion on reddit about it.
Added
From the Wikipedia page, it appears that you could create:
- A
- G
- H
- I
MPer comment: should update Wikipedia so left-hand knows what the right is doing, and vice versa.- N
- O
- P
- U
- W
- X
Of course, it depends slightly on your definition of a 'one-letter' language; does 'M4' count as one letter (and one digit)?
Here's Wikipedia's list of programming languages. From the list, there appears to be some space left for single letter languages. Of course, the Wikipedia list is by no means an exhaustive nor authoratative list. I've noticed that the esoteric language i! is not on the list.
Also, searching on Google for "h programming language" did not lead to a page for a "H" proramming language. (At least not in the first page.)
So, perhaps there are some unclaimed letters in the alphabet, but I suspect that somewhere, there is indeed a language for each letter of the alphabet.
With Unicode, a whole new world opens up for single letter programming languages. Twenty-four for Greek alone and the eastern languages make my mind boggle.
If you're trying to come up with a name for a new programming language, I would point out that single-letter language names yield a very low signal-to-noise ratio in Google. Language names that rely on non-alphanumerics to distinguish them (e.g. C++, C#) are even worse. Adding "programming language" to one's search certainly improves matters some, but removes some of the appeal of the short name.