Nie wahaj się mówić po polsku.

It is a negative imperative: a command or encouragement meaning don’t hesitate.

• wahać się = to hesitate
• wahaj = imperative singular, used when speaking to one person informally
• nie = not
• się = part of the verb

So Nie wahaj się is literally something like Don’t hesitate.

If you were speaking to more than one person, you would say Nie wahajcie się.

Because wahać się is the normal verb meaning to hesitate.

In many Polish verbs, się is a fixed part of the verb and does not always translate directly as yourself.

So:

• wahać się = to hesitate
• not just wahać

If you remove się, you change the verb and the meaning.

That is the normal position in this kind of sentence.

In imperatives, się very often comes after the verb:

• Nie martw się = Don’t worry
• Nie bój się = Don’t be afraid
• Nie wahaj się = Don’t hesitate

So Nie się wahaj would sound wrong.

Because after wahać się, Polish normally uses an infinitive to say hesitate to do something.

• wahać się mówić = to hesitate to speak
• wahać się zapytać = to hesitate to ask
• wahać się wejść = to hesitate to enter

English uses to + verb. Polish just uses the infinitive form directly, with no separate word for to.

Because Polish uses a special pattern for languages after verbs like speak:

• mówić po polsku = speak Polish
• mówić po angielsku = speak English
• mówić po niemiecku = speak German

So po + language form is the normal structure here.

mówić polski is not correct for speak Polish.

It is an adverbial expression meaning in Polish or in the Polish language.

The ending -sku appears in names of languages in this pattern:

• po polsku
• po angielsku
• po hiszpańsku
• po francusku

So polsku here is not the basic adjective polski. It is the form used in this fixed expression.

Because in Polish, names of languages and nationality adjectives are normally not capitalized.

So:

• polski
• po polsku
• angielski
• po angielsku

This is different from English, where Polish and English are capitalized.

Because mówić is imperfective, and it fits the general idea of speaking / using the language.

• mówić = to speak, to be speaking, to speak in general
• powiedzieć = to say, to tell, to say something once and complete it

Here the sentence is encouraging someone to speak Polish in general, so mówić is the natural choice.

Yes, but the meaning changes slightly.

• mówić po polsku = to speak Polish
• rozmawiać po polsku = to converse in Polish / have a conversation in Polish

So:

• Nie wahaj się mówić po polsku = Don’t hesitate to speak Polish
• Nie wahaj się rozmawiać po polsku = Don’t hesitate to converse in Polish

The original sentence is broader and more natural in many situations.

Polish word order is flexible, but the original order is the most neutral and natural:

• Nie wahaj się mówić po polsku.

Other orders are possible for emphasis, but they can sound marked or less natural to a learner.

If you are unsure, keep the original order.

To a group or multiple people informally:

• Nie wahajcie się mówić po polsku.

More formal options include:

• Proszę się nie wahać mówić po polsku.
• Niech się Pan nie waha mówić po polsku.
• Niech się Pani nie waha mówić po polsku.

So the original sentence is the informal singular version.

A rough guide:

• Nie ≈ nye
• wahaj ≈ VA-hai
• się has a soft ś sound, somewhat like a very soft sh
• mówić ≈ MOO-veech with a soft final sound
• po polsku ≈ po POL-skoo

Stress is usually on the second-to-last syllable in Polish words:

• WA-haj
• MO-wić
• POL-sku

Not exactly.

• Nie wahaj się = Don’t hesitate / Feel free
• Nie bój się = Don’t be afraid

Nie wahaj się sounds more like encouraging someone to go ahead without embarrassment or doubt.

Nie bój się focuses more on fear.

You can, but it is more formal and less natural in this sentence.

• mówić po polsku = the normal everyday way to say speak Polish
• w języku polskim = in the Polish language, more formal or academic

So in ordinary speech, Nie wahaj się mówić po polsku is the best choice.