I believe we are not allowed to use another language for communication on Fiverr because Fiverr likely monitors conversations manually. If we were to use a language other than English, it could make it more difficult for Fiverr to catch users who might be planning to deal outside the platform, which is against their terms of service.
This is why Fiverr recommends using English for communication—it ensures transparency and helps Fiverr enforce its rules more effectively. If Fiverr allowed users to freely communicate in any language, it would become easier to bypass the system and arrange transactions outside the website, which is not allowed.
Therefore, even though Fiverr might not explicitly forbid other languages, sticking to English helps maintain compliance with their policies.