From another thread, I was advised to aim several languages in order to raise the revenues of my website. So my first idea in order to achieve that goal was to look for a Wordpress plugin that could handle the multilingual aspect. But it seems that having several languages on the same domain was not SEO friendly. For example, I was wondering what will appear in the list of Google searches?
It seems that the best way would be to have separate domains but that implies double effort in terms of content (no problem with that, the translation will be done manually), but most importantly, in terms of SEO optimization. The second domain will begin from 0, meaning that it can not take advantage from the existing website.

Could you please give me your opinion on that matter?

Thank you in advance!