Can I do SEO for the both languages in one website?

Hello,

I`m working for people who had an online shop created a long time ago. The problem is following: They want to have the website in english now. Their website developers said it`s not possible to do SEO for the both languages in one website so they want to do a second website, which I find really funny. I don`t know any programming languages, etc. that`s why I decided to ask you. Is it really so?

The next thing is that for the shipping rates they have manually calculated the rates for the countries and put it in a excel or similar table instead of putting some dhl plugin or similar plugin. Is this 21 century I ask? Are they really using stone-age platform or people still do this normally?

Thanks in advance.

Hi,
This https://www.dudamobile.com/blog/multi-language-sites-affect-seo/discussing how multi-language websites affect SEO might help you.