Learn Polish: Wielonarodowe projekty rozwijają się szybciej.

It is made by combining wiele (many) and naród (nation). In Polish, compound words are typically joined by the linking vowel -o-. The suffix -owe is then added to turn it into an adjective that matches the plural noun projekty.

Why is wielonarodowe written as a single word without a hyphen?

We write compound adjectives as one continuous word when the first part describes or modifies the second part, meaning (of many nations) in this case. You only use a hyphen when the two parts are equal and independent, such as in polsko-angielski (Polish and English).

Why does the adjective wielonarodowe end in -e in this sentence?

It must agree with the noun it modifies, projekty, which is plural and non-masculine-personal (non-virile). In the nominative case, adjectives describing non-masculine-personal plural nouns end in -e. If the noun were masculine-personal, like pracownicy (employees), it would be wielonarodowi.

Why is the reflexive pronoun się used with rozwijają here?

Without się, the verb rozwijać means to develop or unfold something else, like a company developing a new product. Adding się makes it intransitive, meaning the projects are developing themselves or simply progressing on their own.