Learn Spanish (Spain): Sé hablar inglés. - I know how to speak English.

The accent mark shows that this is the "I" (or yo) form of the verb saber (to know). It distinguishes it from the common pronoun se (which you see in words like llamarse).

Why isn't there a translation for the word "how" in this sentence?

In English, we say "I know how to speak", but in Spanish, you just use a form of saber directly followed by an unconjugated verb (like hablar). You do not add a word for "how" when talking about learned skills.

Could I use conozco instead of sé here?

No. Spanish has two verbs for "to know". You use saber (here as sé) for facts, information, or knowing how to do a skill. You use conocer for being familiar with a person, place, or thing.

Why is inglés written with a lowercase letter?

In Spanish, the names of languages, nationalities, and months are not capitalized unless they appear at the very beginning of a sentence.