two languages website or fresh installation

Hello guys,

I need some consultants about having a one WordPress website with two languages or two separate website and fresh installation for other language.

First of all I want to explain my current situation.

1-the main website has lots of data, something between 10K posts and 2K pages.
2-Its located on main address (mydomain.com)
3-the second language will cover new data, it means data is not same as the first language.
4.I don’t need to connect two data bases to each other.

What are your suggestions according to my situation?

Adding new language to old website?
or
Installing a fresh version on a sub domain (french.mydomain.com?

If I use the second solution what are the advantages and drawbacks?

I would be appreciate if you share your knowledge and experiences with me.

Thanks