Learn Portuguese (Portugal): Os restaurantes baratos não são sujos.

In Portuguese, adjectives usually come after the noun they describe. So, instead of saying 'cheap restaurants' like in English, you say the equivalent of 'restaurants cheap' (restaurantes baratos).

Why do both baratos and sujos end in -os?

Adjectives have to match the noun they are describing in both gender (masculine or feminine) and number (singular or plural). Because restaurantes is masculine and plural, both adjectives take the masculine plural ending -os.

How can I tell that restaurantes is masculine if it ends in -e?

Nouns ending in -e can be either masculine or feminine, so you often have to memorize their gender when you learn them. However, the words around the noun give it away! The masculine plural word for 'the' (os) at the beginning of the sentence is your clue that restaurantes is masculine.

Isn't being dirty a temporary state? Why use são (from the permanent verb ser) instead of estão?

It depends on the context! By using são, the speaker is describing a general, underlying characteristic of these places: they are generally clean establishments. If you used estão, you would be saying that they are temporarily clean right now (for example, right after a deep clean).