Au dessert, la vendeuse nous conseille des pêches, des abricots et un ananas bien sucré.

Au dessert literally means at dessert or for dessert.

It is made from à + le = au:

• à = at, to, for
• le dessert = dessert

In natural English, you would usually translate it as for dessert or when it comes to dessert, depending on context.

Here it sets the scene before the main clause: As for dessert, the saleswoman recommends...

La vendeuse is the feminine form of seller / saleswoman.

• un vendeur = a male seller / salesman
• une vendeuse = a female seller / saleswoman

Because the person is female, French uses la vendeuse.

This is a common pattern in French:

• serveur / serveuse
• coiffeur / coiffeuse
• vendeur / vendeuse

Nous means us here.

The verb conseiller can mean to advise or to recommend. In this sentence, nous conseille means recommends to us or advises us to choose.

French object pronouns usually come before the verb:

• elle nous conseille = she recommends to us / she advises us

So even though English says recommends us... or more naturally recommends ... to us, French places nous before conseille.

Because the subject is singular: la vendeuse.

French verbs must agree with the subject:

• la vendeuse conseille = the saleswoman recommends
• les vendeuses conseillent = the saleswomen recommend

So conseille is the correct third-person singular form of conseiller.

Because the sentence is talking about:

• some peaches
• some apricots
• one pineapple

French uses:

• des for an indefinite plural: some
• un / une for a singular countable noun: a / an

So:

• des pêches = some peaches
• des abricots = some apricots
• un ananas = a pineapple

Yes, un ananas is singular.

The word ananas ends in -s, but that -s is part of the word itself. It is not a plural ending in this case.

So:

• un ananas = a pineapple
• des ananas = pineapples

French has a few words like this where the singular form already ends in -s or another letter that may look unusual to English speakers.

Because sucré is an adjective describing ananas, and in French many adjectives come after the noun.

So:

• un ananas sucré = a sweet pineapple

The word bien here strengthens the adjective:

• bien sucré = nicely sweet / very sweet / good and sweet

So un ananas bien sucré means something like a nice sweet pineapple or a very sweet pineapple.

Literally, bien often means well, but in this kind of sentence it works more like an intensifier.

So bien sucré does not mean well sugared in a literal mechanical sense. It means:

• nice and sweet
• properly sweet
• very sweet

This use of bien is common in spoken and everyday French:

• bien chaud = nice and hot / very hot
• bien mûr = nice and ripe
• bien frais = nice and fresh

Because sucré describes only un ananas, not the whole list of fruits.

The fruits listed are:

• des pêches
• des abricots
• un ananas bien sucré

Only the pineapple is specifically described as sweet, so the adjective agrees with ananas, which is:

• singular
• masculine

That gives sucré.

If the adjective described several masculine or mixed items together, you would use sucrés.

In this sentence, it most naturally describes only un ananas.

French structure strongly suggests:

• des pêches
• des abricots
• et un ananas bien sucré

So the sweetness is attached to the last item.

If the speaker wanted to describe all the fruit as sweet, they would normally phrase it differently, for example by using a plural adjective that clearly applies to the whole group.

French normally keeps the articles in lists like this.

So French prefers:

• des pêches, des abricots et un ananas

rather than dropping the articles.

This is more natural and grammatical in French. English often omits articles in lists more easily, but French usually repeats them, especially with indefinite quantities like des and un.