antlr-language-limit-n-regexp is a variable defined in `antlr-mode.el'.
Its value is

(8192 . "language[ ]*=[ ]*\\(\"?[A-Z][A-Za-z_]*\"?\\)")

Used to set a reasonable value for `antlr-language'.
Looks like (LIMIT . REGEXP). Search for REGEXP from the beginning of
the buffer to LIMIT and use the first group in the matched string to set
the language according to `antlr-language-alist'.

You can customize this variable.