"Oni mówią po angielsku."

Translation: They speak English.

[RyanMatthe12]

What is the difference between oni/one? When should each one be used?

[Latcarf]

"oni" is used when you have a group containing at least one male person, "one" is used otherwise (groups of objects and groups of people with only women and children).

[Allison_Ward]

Is "po" a literal translation to "in" or is it just something you put before a language?

[immery]

It is a preposition, https://en.wiktionary.org/wiki/po#Polish.

Treat it like an expression, po language adjective without i -u

[2Jake2snake]

Why "Mowia?"

[Jellei]

"mówią" is the 3rd person plural of "mówić", "to speak".

[Alex556721]

I heard there was ono as well, is it a group of its. I imagine it will be explained later on.

[Jellei]

"a group of its" would be "one", as "one" is for not masculine-personal plural - ergo, everything that is not 'a group of people with at least one man'.

"ono" is exactly "it" in "he/she/it", but it's used rather rarely. You don't really use it for objects, and if you use it for a child... well, you usually either know the child's name (or at least gender), or just say "dziecko"...