Learn Polish: Chcemy zorganizować dwujęzyczną konferencję. - We want to organize a bilingual conference.

It combines the numeral dwa (two) and the noun język (language). When making compound adjectives from a number and a noun, the first part usually takes an -u- connecting vowel, giving us dwu-. It is written as a single word without a hyphen because the two parts describe a single, unified attribute (bilingual) rather than two independent qualities.

We learned that some compound adjectives use the connecting vowel -o-. Why does dwujęzyczny use -u- instead?

The connecting vowel depends on the word types being combined. When the first part is dwa (two) or trzy (three), the connector is -u-, creating the prefixes dwu- and trój-. The -o- connector is much more common when the first part is an adjective, such as in obcojęzyczny (foreign-language) or długoterminowy (long-term).

If the conference involved three languages, how would we change dwujęzyczną?

You would change the number prefix to represent three, creating trójjęzyczną. Notice that the root for three changes slightly from trzy to trój- when forming these compounds, but the feminine accusative ending -ą remains exactly the same.

Why do dwujęzyczną and have different endings ( vs ) if they agree in case?