You have to keep in mind the occasion, how special this event is, and the audience, how old they are and how they would understand what you're speaking.
answer is B. occasion and the audience
With serious reservations about both the premise of the question and the value of my suggestion, I offer (1) the audience; (2) the occasion. If you care for a discussion of the reservations, let me know by email, and I will post them here as a footnote to my answer.
The Audience (are you writing to friends, or to an academic such as a professor or colleague?)
The Purpose of the content (are you trying to convey intellectual information, or is the material light and casual?)
It is important to consider your audience when deciding to use a formal or informal register of language. Formal language is appropriate for most written communications.
1. What it is that you are writing.
2. If you are writing to somebody then how friendly you are with them.
You should consider the expectations of the people to whom you are talking (or for whom you are writing), and also check your horoscope for the day and see if the astrological influences favor informal or formal English.
Idioms are common in informal context, but they are found in all registers of English usage.
Formal English is THE standard English. This is in oppsoition to informal English which is spoken English and includes slang and colloquialisms.
There is no formal slang. Slang is informal language.
Informal
Two things to keep in mind when you're deciding whether to use formal or informal English are the occasion and the audience.
Consider the audience you are addressing and the level of formality that is appropriate for the situation. Additionally, be mindful of the tone and style you want to convey through your choice of language.
Figures of speech can be used in both formal and informal English. They are often used to add depth and creativity to language, whether in professional writing or everyday conversation.
You should consider the expectations of the people to whom you are talking (or for whom you are writing), and also check your horoscope for the day and see if the astrological influences favor informal or formal English.
occasion and audience
Formal: Goeiedag ----> English equivalent: Goodday Informal: Hoi ----> English equivalent: Hi
Formal English is the English written in works like essays, where it's a standardized, formal way to write it, but informal English is what I'm writing in right now, a loose, not very structured way of writing.
Please = Alstublieft (formal) and alsjeblieft (informal) Thank you = dank u wel (formal) and dankjewel (informal) Goodbye = tot ziens / dag (formal) and doei (informal)
occasion and audience
like the Russian , English is a formal table style .. but it can be formal or informal depending on the diner if he\she wants to.
It means "You (informal) or you (formal)"
Formal English is used in professional settings, academic writing, or formal speeches, while informal English is used in casual conversations, texts, or social media. Formal English follows strict grammar rules and uses sophisticated vocabulary, while informal English is more relaxed with slang, contractions, and colloquial expressions.