You could break it down into something like "it is good with you, wine" or "wine is good with you". At this stage, though, I would just run with it until you're more comfortable with the individual words. You'll be coming across a lot of roundabout translations.
It's hard to directly translate the phrase. 'Is maith' kind of just means 'like'. Leat is the part that means 'you' in the sentence. I suppose a literal translation would be 'Like you wine'. So 'is maith' means 'like' and 'leat' means 'you'.
"Leat" means "with you." The "t" at the end indicates "you" in the second person singular.
"Is maith" basically means "Is good."
"Fíon" is "wine."
"Is maith leat fíon" basically means "Is good, with you, wine;" or in proper English rather than word for word literalism, "Wine is good with you," or "You like wine."
Why does not the word 'is' sound like 'ish' since I is a slender vowel? As it does in 'cáis' for instance?
It is an exception. That's it. Let's just deal with it (some people claim that it used to be spelled "ios"... other people say it's because it can work as an abreviation of "agus"... who knows...)
No, it isn't a question, but rather a statement. It is said as if one knows the person in question drinks / likes wine.
So, is maith leat -means> you like while is maith liom -means> i like; how does it changes when I want to say 'they like' or 'we like'?
The base form of the preposition meaning 'with' is 'le.'
le = with
liom = with me
leat = with you (singular)
leis = with him (or with it when 'it' is masculine)
léi = with her (or with it when 'it' is feminine)
linn = with us
libh = with you (plural)
leo = with them
The 'like' part, 'is maith,' meaning 'is good,' does not change; e.g.,
Is maith liom fíon. = I like wine. (Wine is good with me.)
Is maith leat fíon. = You like wine. (Wine is good with you.)
Is maith léi fíon. = She likes wine.
And so forth.