There are two kind of DNN modules, and the Core localization allows the use of both.
Modules that are translatable and modules that are not Translatable.
The Core HTML module is not translatable, but for instance OpenContent is.
For non-translatable modules, you place a "copy" on the other languages page and link the modules to each other (which is the chain icon what you see in the Localization tab), so DNN knows they belong together.
For a translatable module you place a reference on the other language page and the module handles showing the proper language itself.