When the main menu loads the links it also loads the modinfo.php of each module in that menu *1.
Constants used in a block should be defined in \<yourmodule>\language<thelanguage>\blocks.php. That is loaded when the buildblocks() function is run on each module *2.
Within each module, the main.php language file is loaded automatically by the common.php *3. That is where these constants should be defined instead.