Do accessibility overlay widgets fix this?

No. A floating toolbar does not repair bad markup underneath it. Fix the page.

Does Arabic content need anything extra?

Yes. Right to left layout must be genuine, with correct direction on the page and mirrored spacing and icons. See How do I make my WordPress site work in Arabic? .

Is this expensive to add?

Adding it during design costs almost nothing. Retrofitting it into a finished theme costs far more, which is the argument for raising it in the brief.