Learn Romanian: Prietena mea este româncă.

In Romanian, when you use a possessive word like mea (my), the noun before it must have a definite article attached to it. The base word for female friend is prietenă. To add the feminine definite article -a, you replace the final -ă, making it prietena. You are literally saying 'the friend my'.

Why isn't there a word for 'a' before româncă? Shouldn't it be este o româncă?

In Romanian, you usually drop the words for 'a' or 'an' (un or o) when stating someone's nationality, profession, or religion after the verb este (is). You simply say the person is the nationality, without the article.

How would this sentence change if I were talking about a male friend?

Everything would switch to masculine forms. The sentence would become Prietenul meu este român. Here, prietenul is 'the male friend', meu is the masculine 'my', and român is the masculine word for a Romanian person.

Is româncă an adjective or a noun here?

It acts as a noun here. While English uses 'Romanian' as an adjective in this sentence, Romanian uses specific nouns for people of a certain nationality. Româncă literally means 'a Romanian woman'.