Ta sukienka jest ładna, ale jeden rękaw jest za długi.

Ta means this and it has to agree with the noun it describes.

Sukienka is a feminine singular noun, so the correct form of this is ta.
Compare:

• ten rękaw = this sleeve
• ta sukienka = this dress
• to okno = this window

So ta sukienka means this dress.

Because ładna describes sukienka, and adjectives in Polish must agree with the noun in gender, number, and case.

Since sukienka is feminine singular nominative, the adjective also has to be feminine singular nominative:

• ładny = masculine
• ładna = feminine
• ładne = neuter

So:

• ładna sukienka = a pretty dress
• ładny rękaw = a pretty sleeve

Because rękaw is a masculine noun.

The word jeden means one, but like many Polish words, it changes form depending on gender:

• jeden = masculine
• jedna = feminine
• jedno = neuter

So:

• jeden rękaw = one sleeve
• jedna sukienka = one dress

For the same reason: długi describes rękaw, and rękaw is masculine.

So the adjective must also be masculine:

• długi rękaw = a long sleeve
• długa sukienka = a long dress

In the sentence, jest za długi means is too long, and długi agrees with rękaw, not with sukienka.

Za before an adjective often means too in the sense of excessively.

So:

• za długi = too long
• za krótki = too short
• za duży = too big
• za mały = too small

This is a very common pattern in Polish.

Because there are really two separate statements joined by ale:

• Ta sukienka jest ładna = This dress is pretty
• jeden rękaw jest za długi = one sleeve is too long

In Polish, you normally do not drop jest in this kind of sentence. Unlike in some languages, the present tense of to be is usually still expressed when linking a noun to an adjective.

So the repetition is natural and correct.

Using jeden highlights that one of the sleeves is the problem, not both.

So the sentence suggests:

• the dress is nice overall
• but one sleeve is too long

If you said just rękaw jest za długi, it could still make sense in context, but jeden rękaw is clearer and more natural when you want to contrast one sleeve with the other.

Both sukienka and rękaw are in the nominative singular.

That is because they are the subjects of their clauses:

• Ta sukienka jest ładna
• jeden rękaw jest za długi

The adjectives and determiners match that nominative form:

• ta
• ładna
• jeden
• długi

Because ale means but, and in Polish it is normally preceded by a comma when it joins two clauses.

So:

• Ta sukienka jest ładna, ale...

This is standard Polish punctuation.

Yes, it sounds natural.

It is clear, grammatical, and idiomatic. A Polish speaker would understand it immediately.

A slightly different but also natural version would be:

Ta sukienka jest ładna, ale ma jeden za długi rękaw.

But your original sentence is completely fine and very easy for a learner to analyze.

In careful pronunciation, ę is a nasal vowel, but before certain consonants it is often pronounced in a way that sounds closer to en/em to English ears.

So rękaw is often heard approximately like ren-kav.

A few useful pronunciation notes from the sentence:

• ł sounds like English w
• ę is nasal
• j sounds like English y
• cz and sz are not in this sentence, but Polish often uses digraphs like that

So rough approximations would be:

• ta ≈ tah
• sukienka ≈ soo-KYEN-ka
• ładna ≈ WAD-na
• rękaw ≈ REN-kaf / REN-kav
• długi ≈ DWOO-ghee

These are only approximations, but they are helpful at the beginning.