Learn Dutch: Een beleefde groet is belangrijk in Nederland. - A polite greeting is important in the Netherlands.

The adjective comes directly before the noun groet. Because groet is a de-word (de groet), the adjective gets an -e ending when it follows een.

Then why doesn't belangrijk get an -e at the end?

Because belangrijk is separated from the noun by the verb is. When an adjective describes a noun but comes after a verb instead of sitting right in front of the noun, it always keeps its basic form without an -e.

Why is it just in Nederland and not "in de Nederlanden" like the English "in the Netherlands"?

In English, the country's name is plural and uses "the". In everyday Dutch, the country is just singular and doesn't use an article at all. You just say Nederland.

How could I express this same idea using the indefinite je ("one/people") from this lesson?

This specific sentence avoids a subject pronoun entirely by making "A polite greeting" the subject. To express a similar idea using the indefinite je, you could change the noun into a verb and say: In Nederland moet je beleefd groeten (In the Netherlands, you/one must greet politely).