Learn Polish: Proszę mówić po polsku.

When you want to politely ask someone to do something, you use proszę followed by the infinitive form of the verb. Because proszę literally means "I ask", the sentence translates closely to "I ask to speak."

Why is there a po before polsku?

In Polish, when you talk about speaking, reading, or understanding a language, you must use the preposition po followed by a special adverb form. You can think of po polsku as meaning "in the Polish way." You cannot just use the base noun for the Polish language here.

How would I use this exact same pattern to ask someone to speak English?

You just swap out the language word but keep the po ...-u ending. To ask someone to speak English, you would say Proszę mówić po angielsku.

Is proszę a conjugated verb itself?

Yes! It is the "I" (first person) form of the verb prosić (to ask or to request). So while we translate it as "please" here, the true grammatical structure is "I request to speak in Polish."