About my site theme

I have not installed the theme yet, but I am considering.
The site is with a theme developed from scratch for this client L10N.pt and they do not want the landing page to have that structure.

I designed the Landing page and have it in html, but now I need to integrate it with their site, or leave it as is in HTML.

If I could implement it in WordPress would be better but I have questions:
I am using particular css and js for the landing page I developed (pretty simple).
And they have a multi language site.

So my questions are:
Can I create a template for my landing page and have Custom fields for the text areas so they can be updated by the client and translated to different languages?
And, can I use my own css and js library for this purpose? Where should I save this Styles and scripts for the template I create?
Which Custom fields plugins do you advise me to use to get better results with your Landing page plugin?

Thanks

The page I need help with: [log in to see the link]