Learn Hardcore Indonesian: Teman perempuan saya berjanji datang tepat waktu. - My female friend promised to arrive on time.

It’s ambiguous. Literally it’s “female friend,” and in many contexts it’s understood as a non-romantic friend. For “girlfriend (romantic partner),” Indonesians typically say pacar (gender-neutral). If you need to be explicit, you can say:

Pacar saya (perempuan) = my girlfriend
Teman perempuan saya = my female friend (usually non-romantic unless context says otherwise)

Is berjanji datang correct, or should it be berjanji untuk/akan datang?

All appear in real usage:

berjanji untuk datang = very common and neutral
berjanji akan datang = also fine; “akan” makes the future explicit
berjanji datang = acceptable in everyday speech; slightly more casual/elliptical In careful writing, prefer berjanji untuk datang or berjanji akan datang.

Why isn’t there a subject like dia before datang?

The subject is understood to be the same as the main clause subject (control construction). You could make it explicit:

Teman perempuan saya berjanji (bahwa) dia akan datang tepat waktu.
Teman perempuan saya berjanji untuk datang tepat waktu.

What’s the difference between Teman perempuan saya and Teman saya perempuan?

Teman perempuan saya = “my female friend” (a single noun phrase; female modifies “friend”)
Teman saya perempuan = “my friend is female” (predicate describing the friend) Both are grammatical but mean different things.

Where do possessive pronouns go? Why not put saya before the noun?

In Indonesian, possessives follow the noun: teman saya, rumah saya. With a modifier, the whole noun phrase comes first: teman perempuan saya. You can also use the clitic -ku: teman perempuanku (more informal).

Should I use perempuan, wanita, or cewek?

perempuan: neutral, widely used, respectful
wanita: formal/official contexts, often for adult women
cewek: slang/informal (“girl/chick”) So your sentence with perempuan is safe and neutral. Teman wanita saya also exists (more formal). Teman cewek saya is casual.

Is datang the best verb here? What about tiba, sampai, hadir?

datang: come/arrive (general, very common)
tiba/sampai: arrive (at a destination; slightly more about reaching a place)
hadir: be present/attend (formal events) In your sentence, datang tepat waktu is natural. With a venue you could say datang ke kantor tepat waktu or tiba di kantor tepat waktu. For attendance: hadir tepat waktu.

How do I say “on time” naturally, and where do I put tepat waktu?

Use tepat waktu after the verb:

Dia datang tepat waktu. Alternatives:
nggak terlambat (informal “not late”)
pas waktu (colloquial)
pada waktunya (“in due time,” not always the same as “on time”) Noun form: ketepatan waktu = punctuality.

Indonesian doesn’t mark tense—should I add akan for “will”?

Optional. Berjanji already implies future. Add akan if you want to be explicit:

… berjanji akan datang tepat waktu.

How do I say who the promise was made to?

Add kepada + person:

Teman perempuan saya berjanji kepada saya untuk datang tepat waktu.
Dia berjanji kepada kami bahwa dia akan datang tepat waktu.

What’s the difference between berjanji and menjanjikan?

berjanji = “to promise” (intransitive), often followed by untuk/akan + verb:
Dia berjanji akan datang.
menjanjikan = “to promise (something) to (someone)” (transitive):
Dia menjanjikan bonus kepada karyawan.
For this sentence’s meaning, use berjanji, not menjanjikan.

Can I say Dia janji datang tepat waktu?

Yes, that’s a common colloquial shortening (dropping the ber-). It’s informal. More formal: Dia berjanji untuk datang tepat waktu.

Can I use yang to emphasize who promised?

Yes. Teman perempuan saya yang berjanji datang tepat waktu emphasizes that it was your female friend (not someone else). For contrast you might add: …, bukan adik saya.

How would this change with different pronouns/registers?

Neutral/formal: Teman perempuan saya berjanji akan datang tepat waktu.
Casual with aku: Teman perempuan aku berjanji akan datang tepat waktu. / Teman perempuanku…
Jakarta slang: Teman cewek gue janji bakal datang tepat waktu. (very informal)