Jeg tjekker min ordre i appen, før jeg ringer til kundeservice.

Yes. At tjekke is a very common Danish verb. It comes from English check, but it has been adapted to Danish spelling and pronunciation.

Here:

• at tjekke = the infinitive
• tjekker = present tense

So jeg tjekker means I check or I am checking, depending on context.

You may also see more formal alternatives such as kontrollerer, but tjekker is very natural in everyday Danish.

Because ordre is a common-gender noun in Danish:

• en ordre
• therefore min ordre

In Danish:

• min is used with en-words
• mit is used with et-words

For example:

• en ordre → min ordre
• et nummer → mit nummer

Because Danish normally does not use an article together with a possessive.

So:

• ordren = the order
• min ordre = my order

You do not say something like min ordren in standard Danish.

With apps, Danish usually says i appen when something happens inside the app.

So:

• i appen = in the app

This is the most natural choice here.
På is used in other situations, especially with some websites, platforms, or physical surfaces, but i appen is the normal phrasing.

Because appen is the definite form:

• en app = an app
• appen = the app

Danish usually marks definiteness by adding an ending to the noun. Here, it is a specific app that the speaker has in mind, so appen is used.

Because før jeg ringer til kundeservice is a subordinate clause.

Many Danish writers put a comma before a subordinate clause, as in this sentence. That comma style is called startkomma.

In modern Danish, both of these are acceptable:

• Jeg tjekker min ordre i appen, før jeg ringer til kundeservice.
• Jeg tjekker min ordre i appen før jeg ringer til kundeservice.

So the comma is correct, but leaving it out can also be correct depending on comma style.

Because før introduces a subordinate clause.

In a Danish main clause, the finite verb usually comes in second position. But in a subordinate clause, the word order is different.

So here:

• main clause: Jeg tjekker min ordre i appen
• subordinate clause: før jeg ringer til kundeservice

That is why you get før jeg ringer, not før ringer jeg.

Because Danish often uses the present tense in situations where English might also use:

• the simple present
• the present progressive
• sometimes even a future meaning

So jeg tjekker can mean:

• I check
• I am checking

And the whole sentence can describe:

• a habit
• a usual sequence of actions
• something happening now
• something planned

Danish does not have a separate verb form like English am checking.

When ringe means to call someone by phone, Danish normally uses til.

For example:

• Jeg ringer til min mor
• Hun ringer til lægen
• Vi ringer til kundeservice

So til is the normal preposition after ringe when you mean making a phone call.

Because Danish often uses the bare noun when talking about a service, function, or department in a general way.

So ringe til kundeservice works much like English call customer service.

If you were talking about a specific customer-service department already identified in the conversation, you might see the definite form kundeservicen, but the version without an article is very natural here.

Because each clause needs its own subject.

This sentence has two clauses:

• Jeg tjekker min ordre i appen
• før jeg ringer til kundeservice

Even though the subject is the same person in both clauses, Danish repeats jeg, just like English repeats I in before I call customer service.

Yes. Danish usually writes compound nouns as one word.

So:

• kunde
  • service → kundeservice

This is very common in Danish. English often uses two words where Danish uses one compound word. Other examples are:

• telefonnummer
• ordrebekræftelse
• kundecenter

So kundeservice as one word is completely normal.

Yes. Both can work here.

• før jeg ringer
• inden jeg ringer

Both mean before I call.
Før is very common and simple. Inden is also natural and sometimes feels a bit more explicitly time-related, but in this sentence either one would be fine.