The formal answer is: do an HMVC call to a module controller method, and have that load the language file.
As said before, if you need cross-module calling, you may have a design issue.
If you absolutely insist, \Lang::load() allows you to pass a fully qualified filename (which must start with a / on *nix systems, and with x:\ on Windows). Note that in this case you have to specify the group name, as you can't use "true".