Have you tried HeliBoard? I think it has the best multilingual support for all these FOSS keyboards, but mind you, you have to manually load the dictionaries for the languages you want, and same thing for the swipe libraries, but it’s all documented and linked on the GitHub page.
Probably most, but the issue is that most are phantom accounts. See, if you don’t have a Threads account and someone there searches for you, a phantom account is created, where you’ll receive notifications trying to persuade you to “create” tour Threads account. These accounts inflate the numbers, and its why Threads had such a ridiculously fast ““adoption”” rate