Yes, "vous tous". Because there's also an ambiguity in French, between the "vous" singular (formal) and the "vous" plural, (formal or informal).
The English "you" is originally the exact equivalent of the French "vous". It explains the ambiguity between singular/plural.
But "Vous tous" (vous toutes) is usually used as an emphatic expression. An antonym of vous seul/vous seuls/vous seule/vous seules.
I'm aware of how English lost "thou" and of the influence French had on English after the Norman Invasion. I just hadn't really given much thought to the French side of things beyond what I learned in school learning French as a foreign language. But now that you point it out, it makes sense that "vous" would have the same ambiguity that modern "you" has.
Letters in different languages don't sound the same. A Spanish "ll" is not an English "ll".
It's like when people says that French is impossible to read, because they apply the English rules of "letters-sounding" when they read French. French is very regular in the pronunciation, surprisingly regular, not as regular than Spanish though, that is the champion in regularity.
If you learn all the sounds made by a letter in the language, or a group of letters, you can decide if the sounds are different from the spelling.
Thank you for the correction.
Less than Spanish. Spanish has probably the most regular spelling (among Romance language), it's a language specially made to be phonetical. Spanish and French had completely opposed philosophy in their development. Spanish wanted to be a phonetical language, when French re-latinized and re-greekified many words to sound like an intellectual language, or a language with a great history. (but it also kept naturally the spelling often without a need for re-latinization, to be honest)
The glyphs have no inherent value. Each language that uses the same alphabet assigns letters and letter combinations to sounds differently.
The Roman alphabet was never meant to be used with the Celtic/Gaelic languages. Heck, its ancestor the Phoenician alphabet wasn't meant to be used with Greek or Latin. So when a language adopts an alphabet that doesn't have characters assigned to sounds they have, they have to get creative.
It's the imperative "Be well." So you use "salve" when addressing one person (2nd person singular) and "salvete" when addressing more than one person (2nd person plural).
It literally translates as the imperative "be well". So it's the 2nd person singular "salve" when you're addressing only one person and it's the 2nd person plural "salvete" when you're addressing more than one person.
Here is a plain-English overview of what the cases are and how they work:
Latin cases, in English
Adjectives must agree in gender, number, and case with the nouns they modify, but they have their own declensions. Sometimes you get lucky and the adjective just happens to follow the same declension as the noun, but that is not a guarantee.
Why is the subject "tu" being said here? That is not normal/neutral. Also, why has the word order been consistently an English word order and not a Latin one? Once, when I used a neutral Latin word order, my answer was marked wrong. The order insisted upon was an English neutral word order, not a Latin neutral word order.
The course contributors put the sentences together, and the Duolingo engine breaks them apart and presents them in different forms. If you're asked to provide the correct verb conjugation for "Salve, quis ___ tu?" then the "tu" is an essential clue. But if you're given "Hello, who are you?" then you ought to be able to translate it as "Salve, quis es?"
From now on, whenever you're not sure why something was marked wrong, please either copy&paste or screenshot your full answer so we can help you see the real reason it marked you wrong. If a correctly written valid translation is marked wrong, you can always flag it before moving on to the next prompt and report "My anwer should be accepted." If enough people make that same report, the course contributors will evaluate it.
The verb is "salvere", to be well, and this is the imperative form. Therefore it needs to be conjugated to 2nd person singular or 2nd person plural.
Salvete, Marce et Stephane!