Jeg venter udenfor prøverummet, mens min veninde prøver kjolen.

In Danish, the definite article is usually attached to the end of the noun instead of being a separate word.

• et prøverum = a fitting room

• prøverummet = the fitting room

• en kjole = a dress
• kjolen = the dress

The endings depend on the noun’s grammatical gender:

• -et for many et-words: prøverummet
• -en for many en-words: kjolen

So English uses a separate the, but Danish often uses a suffix.

Because Danish, like German, very often makes compound nouns by joining words together.

• prøve = try / fitting
• rum = room
• prøverum = fitting room

Then the definite ending is added:

• prøverum → prøverummet

English usually writes these as separate words, but Danish often prefers one combined word.

Veninde means female friend, while ven usually means male friend.

So:

• min ven = my male friend
• min veninde = my female friend

If the speaker is talking about a female friend, veninde is the natural choice.

Danish does not have a separate present continuous form like English am/is/are + -ing.

So the Danish present tense can cover both:

• Jeg venter = I wait / I am waiting
• min veninde prøver kjolen = my friend tries the dress / my friend is trying on the dress

The context tells you that the action is happening right now.

Because Danish prøve can already mean try on when the object is a piece of clothing.

So:

• Hun prøver kjolen = She is trying on the dress

You do not have to add a separate word meaning on.

You can also hear:

• Hun prøver kjolen på
• Hun prøver den på

But in a sentence like this, prøver kjolen is completely natural.

Danish vente can be used on its own when the context already makes it clear what is happening.

• Jeg venter. = I’m waiting.

If you want to say what or who you are waiting for, Danish often uses på:

• Jeg venter på min veninde. = I’m waiting for my friend.

In your sentence, the location and the mens-clause already explain the situation, so Jeg venter udenfor prøverummet sounds natural.

Mens means while.

It connects two actions happening at the same time:

• Jeg venter udenfor prøverummet
• mens min veninde prøver kjolen

So the full sentence means that one action is happening while the other is happening.

Yes. Mens introduces a subordinate clause.

In the clause after mens, the order here is:

• mens
  • subject
    • verb
      • object
• mens min veninde prøver kjolen

That is normal Danish subordinate-clause order.

A useful thing to notice is that if you move the mens-clause to the front, the main clause changes word order:

• Mens min veninde prøver kjolen, venter jeg udenfor prøverummet.

Not jeg venter after the comma, but venter jeg, because Danish main clauses normally keep the finite verb in second position.

Because mens introduces a subordinate clause, and in standard written Danish that clause is separated with a comma.

So:

• Jeg venter udenfor prøverummet, mens min veninde prøver kjolen.

That comma is normal and expected in writing.

Here udenfor means outside.

It tells you where the speaker is waiting:

• Jeg venter udenfor prøverummet = I’m waiting outside the fitting room

So it functions as a location word.

Because kjolen means the dress, a specific dress that both speaker and listener can identify from the situation.

In this context, the friend is trying on a particular dress, not just any dress in general.

Compare:

• Hun prøver en kjole. = She is trying on a dress.
• Hun prøver kjolen. = She is trying on the dress.

So the definite form makes sense because the dress is specific.

Yes, absolutely. That is a very natural alternative.

The meaning stays basically the same, but the emphasis changes slightly because now the while-clause comes first.

The important grammar point is this:

• Mens min veninde prøver kjolen, venter jeg udenfor prøverummet.

After a fronted clause, Danish main clauses usually put the finite verb before the subject:

• venter jeg
• not jeg venter

So this is a good example of normal Danish V2 word order.