antlr-language-alist is a variable defined in `antlr-mode.el'.
Its value is

((java-mode "Java" nil "\"Java\"" "Java") (c++-mode "C++" "\"Cpp\"" "Cpp"))

List of ANTLR's supported languages.
Each element in this list looks like

MAJOR-MODE, the major mode of the code in the grammar's actions, is the
value of `antlr-language' if the first group in the string matched by
REGEXP in `antlr-language-limit-n-regexp' is one of the OPTION-VALUEs.
An OPTION-VALUE of nil denotes the fallback element. MODELINE-STRING is
also displayed in the mode line next to "Antlr".

You can customize this variable.