El meu germà diu que l'armari de dalt és ple, però el de sota encara és buit.

In standard Catalan, a possessive usually goes together with a definite article:

• el meu germà
• la meva germana
• els meus llibres

That is different from English, where you normally just say my brother without the.

So el meu germà is the normal everyday pattern.
You may also come across forms like mon germà, but those are more literary, regional, or traditional.

Because el becomes l' before a word starting with a vowel or silent h:

• el + armari → l'armari
• el + hotel → l'hotel

This is just the normal apostrophe rule in Catalan.
It works much like French or Italian elision.

Here que introduces the clause after diu:

• El meu germà diu que...
• My brother says that...

In English, you can often omit that.
In Catalan, que is normally kept, so diu que is the standard form.

They are very common location expressions:

• de dalt = the one above / the upper one / on top
• de sota = the one below / the lower one / underneath

So:

• l'armari de dalt = the cupboard/wardrobe above
• el de sota = the one below

The exact English translation depends on the situation, but the basic idea is upper vs lower.

Because Catalan, like English, can leave out a repeated noun when it is already clear from context.

So:

• l'armari de dalt = the upper cupboard
• el de sota = literally the one below

Here el stands in for the omitted masculine noun armari.

Compare:

• la cadira de dalt i la de sota = the chair above and the one below

So this is a very natural way to avoid repetition.

Because they agree with armari, which is masculine singular.

• l'armari ... és ple
• el de sota ... és buit

Even in el de sota, the noun armari is still understood, so the adjective still agrees with it.

If the noun were feminine, the adjectives would change:

• la capsa és plena
• la de sota és buida

So the pattern is:

• masculine singular: ple, buit
• feminine singular: plena, buida

This is a very common learner question because Catalan does not use ser and estar exactly like Spanish or English.

In Catalan, ser is often perfectly natural with adjectives like ple and buit, especially when describing something such as a room, cupboard, bottle, etc.:

• L'armari és ple
• L'ampolla és buida

So és ple and és buit are completely normal here.

You may also hear està ple or està buit in some contexts, especially if the speaker wants to emphasize a temporary state or resulting condition, but és ple / és buit is very standard and natural.

Here encara means still:

• encara és buit = is still empty

It shows that the situation continues up to now.

A useful related pattern is:

• encara no = not yet

For example:

• L'armari encara no és ple = The cupboard isn’t full yet

So in your sentence, encara emphasizes that the lower cupboard remains empty.

Sometimes yes, but the nuance is a little different.

• de sota usually means underneath / below
• de baix usually means down below / lower / downstairs

When two things are vertically arranged, de dalt and de sota are a very natural pair.
For stacked cupboards or shelves, de sota sounds especially appropriate.

So l'armari de dalt ... el de sota is a very idiomatic choice.

They have accents for spelling reasons.

• germà has an accent because the stress is on the last syllable: ger-MÀ
• és has an accent to mark the verb form is and to distinguish it from es, which is a different word

So the accents are not optional; they are part of the standard spelling.

The sentence as written is very natural:

• El meu germà diu que l'armari de dalt és ple, però el de sota encara és buit.

Catalan does allow some word-order flexibility, but changing it usually changes emphasis rather than basic meaning.

For example, moving encara can slightly shift the focus, but encara és buit is the most straightforward choice here.

So yes, Catalan has some flexibility, but this sentence uses a very normal, neutral word order.