A mi m'agrada amanir l'enciam amb llimona, però a la meva amiga li agrada més el vinagre perquè diu que no és tan àcid.

Because A mi adds emphasis or contrast. The sentence is comparing two people:

• A mi m'agrada...
• però a la meva amiga li agrada més...

So A mi here feels like As for me or Me, I....
You could say just M'agrada amanir l'enciam amb llimona, but the full version makes the contrast with the friend clearer.

Agradar does not work like English to like. It works more like to be pleasing to.

So:

• M'agrada amanir l'enciam amb llimona
  literally = Dressing lettuce with lemon is pleasing to me

The person who likes something is expressed with an indirect object pronoun:

• m' = to me
• li = to him/her

And the thing being liked is the grammatical subject of the verb.

Because the pronouns are different:

• m' = em = to me
• li = to him / to her

So:

• A mi m'agrada = I like
• A la meva amiga li agrada = my friend likes

The apostrophe in m'agrada happens because em becomes m' before a vowel:

• em agrada → m'agrada

Because the thing being liked is the whole action amanir l'enciam amb llimona.

An infinitive phrase like amanir l'enciam amb llimona is treated as one single idea, so the verb stays singular:

• M'agrada amanir l'enciam...

Compare:

• M'agrada cuinar = I like cooking
• M'agraden les pomes = I like apples

So if the subject is an action, agrada is normal.

Amanir means to dress, to season, or to add dressing/condiments to food, especially salad or vegetables.

Here amanir l'enciam amb llimona means something like:

• to dress the lettuce with lemon
• to season the lettuce with lemon

It is a very natural verb in food contexts.

There are two different things happening here.

1. l'enciam
   This is the definite article el shortened before a vowel:

• el enciam → l'enciam

2. amb llimona
   Here there is no article because Catalan often leaves out the article when talking about an ingredient or substance in a general way:

• amb llimona = with lemon
• amb vinagre = with vinegar

If you said amb la llimona, it would sound more like with the lemon, meaning a specific lemon.

Also, llimona starts with ll, which is a consonant sound, so it does not take apostrophe:

• la llimona, not l'llimona

In Catalan, possessives normally go together with the definite article:

• la meva amiga = my friend
• el meu germà = my brother
• les nostres cases = our houses

This is different from English, where you usually do not use the before my.

So la meva amiga is the normal Catalan structure.

It may look repetitive to an English speaker, but it is very normal in Catalan.

• a la meva amiga names the person explicitly
• li repeats that person with the indirect object pronoun

This is called clitic doubling, and Catalan uses it a lot, especially with people:

• A la Maria li agrada el cafè
• Al meu pare li encanta cuinar

So a la meva amiga li agrada is standard and natural.

Here més means more.

So li agrada més el vinagre means:

• she likes vinegar more
• or more naturally, she prefers vinegar

The idea is that, compared with lemon, vinegar is what she likes more.

The word order is also normal in Catalan. The thing liked often comes after the verb:

• li agrada més el vinagre

Here perquè means because:

• perquè diu que... = because she says that...

This is different from per què, which usually means why or for what reason:

• Per què ho fas? = Why are you doing it?

So:

• perquè = because
• per què = why / for what reason

This is a very common spelling distinction in Catalan.

The subject is understood from context: it is the friend.

Catalan often leaves out subject pronouns when they are not needed. So instead of saying ella diu, Catalan often just says diu.

In this sentence:

• a la meva amiga li agrada més el vinagre perquè diu que...

it is naturally understood as:

• because she says that...

So the speaker does not need to repeat ella.

Tan means so or as. In a negative sentence, no és tan àcid often means:

• it isn't so acidic
• it isn't as acidic

The comparison is implied: vinegar is, in her opinion, less acidic than lemon.

Àcid is masculine because it agrees with el vinagre, which is masculine:

• el vinagre és àcid
• la llimona és àcida

So if the adjective referred to llimona, it would be àcida, not àcid.