Sorry to say that but your instructions are not clear enough about the installation way of the .mo and .po translation files. I've tried to copy them (FR version) to the following folders:
wp-content/plugins/simplemodal-login/lang (I created this folder)
wp-content/plugins/simplemodal-login/languages (I also created this folder)
None of them changed the default english language ...
Please advise which folder must be used and PLEASE put the advice on your FAQ (I searched for hours ...!!!).