Learn Swedish: Ni är långsamma idag.

Because it is describing ni, which is plural (meaning "you guys" or "y'all"). In Swedish, when an adjective describes a plural subject, you must add an -a to the end of it. The base word for "slow" is långsam.

Why does långsamma have a double m when the base word långsam only has one?

This is a spelling rule to keep the vowel sound short. In långsam, the second vowel is short. When we add the plural -a, we have to double the m to -mm- (långsamma) so the previous vowel doesn't accidentally turn into a long sound.

How would I say this if I were only talking to one person?

You would change the pronoun to du (singular "you") and remove the plural -a ending from the adjective: Du är långsam idag.

Is idag written as one word or two words?

It can be both! You will see it written together as idag or separated as i dag (literally "in day"). Both are completely correct and mean "today", though i dag is technically recommended by official spelling guides.