Please note that this just translate the main Mosaico UI: some parts of the UI are defined by the template and their "labels" cannot be translated by the library because each template defines its own labels/texts. So you may want to create your own "translated" templates by altering the "source" html for the template.
You can even force mosaico to run the internal translation tool for the template labels by defining a [plugin](https://github.com/voidlabs/mosaico/wiki/Mosaico-Plugins):