YOU IN ENGLISH
How do english people understand "you" as formal , informal or plural
English does not have the same system of formal and informal pronouns. For very formal times, you might say "How are you today, Sir?" and make it more formal by adding another word like "Sir," "Miss," or "Ma'am"" (depending on gender and age.)
As for how we know a noun is plural, we understand from the verb form and context. Honestly it can be confusing sometimes, but from context you should know if a group or person is being referred to.
We grew up with only 'you' for all occasions, so for us it is normal.
To indicate formal when speaking with someone, we will likely use formal titles. E.g. Teachers are 'Miss,' 'Sir,' 'Mrs. [last name]', etc.
Talking American English, we sometimes add words to 'you' for plural: you all, y'all (contraction of you + all), you guys, etc.. E.g. "Are you all coming?" "Y'all have any questions?" "Where are you guys going?" Also, sometimes 'you' is replaced in the plural with 'everyone'/'anyone'. E.g. "Is everyone OK?" "Anyone coming?"
Hope that helps.
In English you express politeness in other ways.
Passe-moi le sel (s'il te plaît). // Pass me the salt (please).
Passez-moi le sel s'il vous plaît. // Could you pass me the salt, please?
Pourriez vous me passer le sel s'il vous plaît? // Would you be so kind as to pass me the salt, please?
It comes down to slang or colloquial language. In Australian English some people say "yous" for the plural of you, and "ya" for an informal version of you. Certain British/Irish dialects share these traits.
One could use these informally but not at work for example, because they aren't considered "proper English", and you probably wouldn't use them with people significantly older than you either.
Historically, English used to have the informal (familial) you form: thee/thou. (Old enough that it's from when English was still inflected, I believe.) Many people think that thee/thou (and thine, with 'art' for the 'to be' form) are more formal, because the only places people hear them are in prayer form - but since prayers were directed to 'Our Father' - they use the familial, informal form.
It's not used in everyday speech though, and people really will look at you funny if you use it!
In modern English, 'you' is neither formal or informal, and is both singular and plural. Northern Irish dialects do have 'yous' in spoken form for plural 'you', and some Southern US dialects use 'y'all' in a similar way.
'Yous' is used by speakers of some varieties (dialects) of English, but it is not used generally outside the community which uses that variety of English. For example, in an international English language examination you would almost certainly lose a mark for using the word 'yous', since it would not be regarded as 'standard English'.
You can add "all" if you need to emphasize that "you" refers to more than one person. There are different ways of phrasing it, but it is usually safe to replace "you" with "you all".
Using "y'all" or "yous" may be endearing if you're in an area where they are used frequently (such as the southern USA for "y'all"), but to everyone else, it will sound very strange, especially coming from a non-native speaker.