En la ferio ni volas provi ĉiajn manĝaĵojn, sed mia fratino ankoraŭ preferas panon kaj supon.

Both can work, but they are slightly different in feel.

• en la ferio = in the vacation / on the holiday
• dum la ferio = during the vacation

In everyday Esperanto, en la ferio is a normal way to say that something happens within that holiday period.
dum la ferio stresses the time span a bit more explicitly.

So in this sentence, En la ferio is a natural way to mean while we're on vacation.

In Esperanto, ferio is often used as a singular noun meaning a holiday period or vacation.

So:

• la ferio = the vacation / the holiday period
• ferioj = holidays, vacations, or several holiday periods

Even if English uses a plural form like the holidays, Esperanto may still use the singular.

After volas (want), Esperanto normally uses the infinitive form of the next verb.

• voli = to want
• provi = to try

So:

• ni volas provi = we want to try

This is similar to English want to try.
The infinitive in Esperanto usually ends in -i.

ĉiajn means all kinds of or various kinds of.

It comes from ĉia:

• ĉia = of every kind, of all kinds

So:

• ĉiajn manĝaĵojn = all kinds of foods / all sorts of dishes

This does not mean exactly the same as all the foods in a complete list. It emphasizes variety.

Because this whole phrase is the direct object of provi.

They want to try the foods, so the object must take -n.

Also, both words must agree:

• ĉiajn
  • -j because plural
  • -n because accusative
• manĝaĵojn
  • -j because plural
  • -n because accusative

So:

• ĉiaj manĝaĵoj = all kinds of foods (as a subject or basic noun phrase)
• ĉiajn manĝaĵojn = all kinds of foods (as a direct object)

Yes, you could say ĉiujn manĝaĵojn, but it means something different.

• ĉiajn manĝaĵojn = all kinds of foods, many different types of food
• ĉiujn manĝaĵojn = all the foods, every single food in the relevant group

So ĉiajn focuses on variety, while ĉiujn focuses on the whole set.

In this sentence, ĉiajn is more natural because the idea is that they want to try many different kinds of food on vacation.

manĝaĵo means a food item, something to eat, or a dish/food.

It is built from:

• manĝi = to eat
• -aĵ- = a concrete thing made from or related to the root

So:

• manĝaĵo = food item / edible thing
• manĝaĵojn = food items (direct object, plural)

Using manĝaĵoj is very common when talking about foods or dishes.
manĝoj would more naturally suggest meals in some contexts, not necessarily types of food.

Because they are also direct objects, this time of preferas.

• preferi ion = to prefer something

So in:

• mia fratino ankoraŭ preferas panon kaj supon

the sister prefers:

• panon = bread
• supon = soup

Both are objects, so both take -n.

Because mia fratino is the subject, not the object.

She is the one doing the action:

• mia fratino ... preferas = my sister prefers

In Esperanto:

• the subject normally has no -n
• the direct object takes -n

So:

• mia fratino = subject
• panon kaj supon = direct objects

Here ankoraŭ means still.

So:

• mia fratino ankoraŭ preferas panon kaj supon = my sister still prefers bread and soup

It suggests a contrast: even though we want to try all kinds of foods, she still likes the simpler, familiar things.

Depending on context, ankoraŭ can also mean yet or still, but here still is the best fit.

In Esperanto, adverbs like ankoraŭ are often placed before the word they mainly modify.

So:

• ankoraŭ preferas = still prefers

That is a very natural word order. Esperanto word order is fairly flexible, but this placement is clear and common.

Here it most naturally means bread and soup in a general sense.

Esperanto often uses nouns without an article when speaking generally:

• panon = bread
• supon = soup

If you wanted to make them more specific, you could use la:

• la panon kaj la supon would mean the bread and the soup

But in this sentence, the general meaning is more natural: she prefers bread and soup as types of food.

Yes. preferi is the standard and most direct verb for to prefer.

So:

• mi preferas teon = I prefer tea
• ŝi preferas supon = she prefers soup

A learner might wonder whether Esperanto would instead use something like ŝati pli (to like more). That is possible in some situations, but preferi is the normal and simplest choice here.

A few useful ones from this sentence:

• ĉ = like ch in church
  • ĉiajn
• ĝ = like j in jam
  • manĝaĵojn
• ĵ = like the s in measure
  • manĝaĵojn
• ŭ = a short w-like sound, though it does not appear here as a separate letter in this sentence

So ĉiajn manĝaĵojn has several sounds English speakers often need to practice carefully.