Tonga sits in Oceania, and that tends to shape what comes up — food, sport, weather, the working week. Because nobody is filtered by border, though, the next skip might drop you into Tuvalu or New Zealand instead, with a completely different set of reference points.
Tongan is what you will hear locally, but English is official in Tonga and turns up constantly in everyday speech. Expect a mix, and expect idioms that will not appear in any textbook.
Time zones do most of the work in deciding who is awake. Tonga runs on UTC+13, so its 9pm is 6pm in Sydney and 9am in Lagos, and evenings are when conversations stretch past the first question.
Nothing at all. Random matching, video, voice and text are open to everyone in Tonga, whether the conversation runs in Tongan or English, and there is no trial that expires or premium tier holding back the useful parts.
Signing up is not part of it. Anyone in Tonga can open the page and chat anonymously in Tongan or English; the age confirmation is the only box to tick, and nothing you type becomes an account.
English is safe to open with, since it sits alongside Tongan in official use in Tonga. A few words of Tongan still change the tone of a chat for the better.
Yes — timing changes the mix more than anything else. Tonga runs on UTC+13, and its 9pm is 6pm in Sydney and 9am in Lagos; outside that window you are likelier to meet Oceania neighbours who are still awake.