"y'all" fills a legitimately useful gap the English language has. Other languages have a word like this.
Edit: also something cool I just found out, some languages have a way to disinguish "we" (you and I), and "we" (me and the rest of us, not you). It's called clusivity and is missing from European languages. Many indigenous languages of the Americas and Oceania have this, as well as Vietnamese and northern dialects of Mandarin.
"Vous" is the first one that comes to mind in french. But since it is also a more formal (and/or "respectful") version of "tu/toi", it can both designate a group of people or a single person, depending on the context (just like "you" in English). Sometimes people will use "vous tous" (literally "you all") to make this clear.
It is a little better than the "you" situation in English since if you are speaking with someone that is not using the singular form of "vous" to speak about you (which is basically anyone you are familiar with unless they are your boss or In-laws and kind of oldschool), it is instantly clear what they mean at least.
In Portuguese (especially Brazilian), there are singular and plural forms of "you": "você" (singular) and "vocês" (plural). In English, "you" behaves like a plural because it's followed by "are" instead of "is". The only exception I can see is "yourself" and "yourselves" that refer to both singular and plural forms.
However, In Portuguese, even though we have "vocês" as plural form, we also use "vocês todos" or "todos vocês" ("you all"/"all of you") sometimes.
English used to be like other European languages too. We had thou/thee for singular, and you/ye for plural, and for respectful singular. Eventually, people began using it as respectful singular for everyone, and so it just became singular and plural, eclipsing thou/thee. Around this time, the you/ye accusative/nominative distinction was also lost, so now we just have you.
If you're curious, the you/ye distinction worked like this: "you" was used for the subject (the doer) of the sentence, and "ye" was used for the object (the done to). you/ye are analogous to I/me.
"You come with me." (plural you)
"I come with ye." (plural ye)
As a result of the loss of thou, we also lost the conjugation of verbs related to it, like "art" instead of "are", and "-st" or "-est" for other verbs ("goest", "thinkst", etc). It used to be that "are" was only for plural pronouns, but now both "you" and "they" can be singular.
And if you're curious about what happened to "-eth", evidence suggests this was for a long time a typographic feature, and it was pronounced "-s" as it is today. It was used exactly like "-s". "He thinketh" would have been pronounced "he thinks".