English has a lot of words specific to boats, like galley, head (not the kind you need to use to learn a language), starboard, the list goes on.
Question about Czech informal: does the fact that there's so much informal mean that the Czech are very lax about what doesn't constitute formal? What would constitute formal in the Czech Republic?
Formal and informal is very complicated for people that come from languages that are missing them. Way easier to explain to a Frenchman, well, really no need to explain to them :D.
Typically informal is any family member (not necessarily true in the history though), friends, little kids or teenagers even if you do not know them, kids among themselves. Formal is everybody else. Once you start what czechs call middle school but is equivalent of a high school in US, at about 14 years of age, teachers switch to formal addressing of the kids. You are formal with anybody you do not know, people on the street, in stores, at work. That can change after a while and one or the other party can offer 'informality'. That has rules of its own. Woman offers it to man, boss to employee, older to younger, etc. It can get little awkward at times. At other occassions you jump straight into informal, let's say meeting your best friend's husband or close friends when you all are heading camping for the weekend, you would just most likely start informally from the get go. The 'switch' from informal to formal is often toasted to (with alcohol of course).
The informal/formal is not only greetings and such, it is the entire use of verbs with which you address the other party. Same structure as French, German (though they use 3rd person, but the idea is the same) and I suspect Spanish.
Bohužel, záleží kde a v čem. Hello je dost často přeloženo jako zdravím, nikoliv ahoj, záleží i na kurzu a Hi teprve jako "ahoj" Ahoy (stejná česká výslovnost) se chápe jako vodácký pozdrav, zdomácněl. Ale v BE je to výhradně námořní pozdrav, pamatuji si " ahoj člun" z Tajuplného ostrova, klukovská četba, a tam bla k tomu i vysvětlivka. Předpokládám, že v AE rovněž. V kurzu ruštiny Duo tohle přísně dodržuje.