Well, it is a hard question. And the best person to answer it are you. I like simple licenses (BSD[1], MIT[2]...), but only if you don't mind people making closed version of it.

Being Arc a language, I think you would like to remain the head of its development, so maybe the Artistic License [3][4]would be right. Like Perl does.

It's far more complicated than the simple ones, but it would make easier to differentiate between the Vanilla Arc and the Straciatella Arc.

For the GPL[5], I think it is better suited for programs than for languages.

[0] http://www.opensource.org/licenses/category

[1] http://www.opensource.org/licenses/bsd-license.php

[2] http://www.opensource.org/licenses/mit-license.php

[3] http://www.opensource.org/licenses/artistic-license.php

[4] http://www.opensource.org/licenses/artistic-license-2.0.php

[5] http://www.opensource.org/licenses/gpl-license.php