Learn Polish: On mówi coraz wolniej. - He is speaking slower and slower.

Both structures use comparatives, but they do different things. While im..., tym... compares two changing things against each other, coraz just shows that a single thing is continuously changing over time. You combine coraz with a comparative word to mean "more and more". So coraz wolniej perfectly captures the English repeating phrase "slower and slower".

In English we repeat the comparative ("slower and slower"). Could I translate that literally as "wolniej i wolniej"?

You will be understood, but it sounds very unnatural to Polish ears. Polish almost always uses coraz plus the comparative just once, instead of repeating the word with an "and" like we do in English.

Why is "slower" translated as wolniej here instead of wolniejszy?

It depends on what you are describing. Because "slower" describes the verb mówi (how he is speaking), you need the comparative adverb wolniej. If you were describing a noun, like a slower person or a slower car, you would use the comparative adjective wolniejszy.

Since there is no "is speaking" tense in Polish, how do we know the sentence means he is doing it right now?

Polish imperfective verbs like mówi cover both "he speaks" and "he is speaking". However, adding coraz wolniej describes a gradual, active change. This context makes it very clear that it is an ongoing action happening in the moment.