Translation questions


Hi. I’m customizing the Flatsome theme and I have some questions about translations on wordpress.

First of all, I want to translate
–>posted on XXX, by XX
–> archived: Uncategorized
Every automatic generated text in the blog is in English. Where can I translate it ?

If I need to custom .php files, will it be possible to translate these pages to have the french and english versions on my website ?

Like I also want to translate the .404 error page. But if I translate it in the CSS editor, will it be possible to have 2 versions (French and English) of this page?

Thanks for your help !