I completely agree that the issue of languages is critical and needs to be solved now, or you’ll end up with something like the U.N. and it’s myriad of official and working languages and the incredible amount of energy and money and time that’s poured into translations. I’m keeping in mind that machine translation is going to keep improving, and can be used to automate a lot of the basic language tasks, but I think it would be unwise to depend on it.
But I think you might have a problem if you want to try to replace a national language with some other national language. National languages carry baggage (historical, cultural, ideological), and most of them (not all, but most) carry such a high mental overhead in terms of grammar, vocabulary, and grammatical exceptions that it’s just, well, hard to learn a new language, especially if that’s not really your passion. People who are born to the language will have a natural advantage – people born on top of the mountain (whether it be a sharp, high peak like English or Chinese or a gentler slope like, say, Spanish) are already there, and the rest of us have to climb this peak.
Granted, I think that Seasteaders would have more motivation than most to do it. But I doubt that the people who are going to be able to afford this (at least in the beginning) are going to be inclined to put all that effort into learning a new language, even if we can avoid the standard arguments that boil down to “how very dare you insist that your language better as a common language than my language”.
An alternative would be to have an auxiliary language: everyone continues to speak their own languages, but has an easy-to-learn “hip pocket” language that they can pull out of their pocket when they encounter someone who doesn’t speak their own language. This is what Esperanto was designed to do, and it is demonstrably easier to learn than “natural” (national) languages. Lots of people poo-poo it, saying things like “it’s a dead language” (not true), or “we already have an international language, it’s English” (I’d argue that, China notwithstanding, what I call “Scripted English” is a better name for the “English” that is “international”), but it’s insanely flexible, rapidly acquired, and useful. Plus, since it’s artificial, it doesn’t have the same cultural baggage as a national language.
In short, if Seasteaders are really serious about having a common language, give Esperanto a try.