Since you can write down all abstract syntax trees using the same syntax, and you can convert most languages into an abstract syntax tree (all the popular ones), it would be easy enough to write down most languages using the same syntax.

The real question would be whether that AST syntax would be better than the language's own syntax. I'm guessing it would be harder to read, since a good syntax for an AST would describe only the semantics of the tree, making it harder to pick out the structure of the language itself.