The plugin does not provide multilingual translation of dynamic content (i.e. custom dialog messages, email content, etc). There is simply no way for that to work since once you change the message (which you should do to customize to your use) it would no longer match a string in a static .po file and/or compiled .mo.
Bojan is correct – customize the message in the emails tab.
If you need something multilingual, then you probably would get the best results by filtering the email content based on what language you need. For this you can use the wpmem_email_filter filter hook. This filter is passed pretty much every piece of the email in an array, as well as additional arrays of the registration fields and the user’s registration data. From this, you can customize any aspect of the message.
Should I do edit some php codes?
I would advise against that. If you do that, you will break your setup every time you update the plugin. The correct way to approach this is with a custom filter.
