The language string container is global, so it doesn't matter from where you load a language file, you always access them the same way. They are not prefixed with a module or anything.
You can not load a language file from a specific module, modules are supposed to be self-contained, so only access the data in the module from a controller inside the module. If you really need it, the proper way is to do an HMVC call to the module to load it.
The workaround is to specify a fully qualified filename: APPPATH.'modules/mymodule/lang/en/actions.php and use that to load. But it is not recommended practice.