Learn Hardcore Indonesian: Jangan sampai salah belok di depan penyeberangan. - Do not let yourself turn the wrong way in front of the crosswalk.

Jangan = a direct “Don’t …” command.
Jangan sampai = “Make sure you don’t / Don’t let it happen.” It warns against an unwanted outcome, often accidental. It’s slightly softer in tone but emphasizes avoiding the consequence (ending up taking a wrong turn).

Where is the subject? Who is being told not to do this?

It’s implied as “you.” Indonesian often drops pronouns when context makes them clear. If you want to include one:

Casual: Jangan sampai kamu salah belok…
Polite/neutral: Jangan sampai Anda salah belok…
Plural: Jangan sampai kalian salah belok…
Respectful address: Jangan sampai Bapak/Ibu salah belok…

Does sampai here mean “until”?

No. In Jangan sampai + clause, sampai means “to the point that / end up.” It’s about preventing an outcome, not marking time. Compare: Jangan sampai ketinggalan (Don’t end up missing it).

Is salah belok a natural phrase? Why not belok yang salah?

Yes—salah + verb is a very common pattern meaning “to do X wrongly”: salah tekan (press the wrong button), salah kirim (send to the wrong person), salah belok (take a wrong turn).
Belok yang salah sounds odd unless you’re treating it as a noun phrase with a very specific context. A more natural nouny alternative is ambil belokan yang salah (“take the wrong turn”), but salah belok is shorter and very idiomatic.

What’s the difference between belok, membelok, and membelokkan?

belok: intransitive “to turn” (most common in speech).
membelok: also “to turn,” more formal/written; still intransitive.
membelokkan: transitive/causative “to make something turn; to divert” (e.g., membelokkan mobil ke kiri, “turn the car to the left”; membelokkan pembicaraan, “steer the conversation”).

Could I say Jangan salah jalan instead? What’s the difference?

salah belok = a specific wrong turn at a point.
salah jalan = the wrong route/direction more generally.
Use the one that matches your meaning.

What exactly does di depan mean here—“in front of” or “before”?

In road directions, di depan means “ahead of / in front of (along your path).” It can overlap with “before (reaching)” in English. If you want it unambiguously sequential, say:

sebelum penyeberangan = before (you reach) the crossing
menjelang penyeberangan = approaching the crossing

What does penyeberangan refer to here? Is it a pedestrian crossing?

In road context, yes—usually a pedestrian crossing/crosswalk. You can say penyeberangan jalan or penyeberangan pejalan kaki for clarity. In other contexts it can mean a ferry crossing (penyeberangan sungai/laut), so context matters.

I often see penyebrangan/menyebrang. Are those spellings wrong?

Standard spelling is penyeberangan/menyeberang (KBBI). The forms without the second “e” (penyebrangan/menyebrang) are very common informally and on signage but are nonstandard. Morphology: base seberang → meN- + seberang → menyeberang; peN- + seberang + -an → penyeberangan.

Can I move the place phrase to the front or middle, like Jangan sampai di depan penyeberangan salah belok?

The natural order is as given, with place at the end: Jangan sampai salah belok di depan penyeberangan.
Fronting for emphasis is okay: Di depan penyeberangan, jangan sampai salah belok.
The middle position you proposed sounds awkward.

How do I add the direction (left/right)?

Add it after belok:

Jangan sampai salah belok ke kiri di depan penyeberangan.
Jangan sampai salah belok ke kanan di depan penyeberangan.

Is the tone polite enough for strangers? How can I soften it?

It’s acceptable, but you can soften it:

Add tolong: Tolong jangan sampai salah belok…
Add a warning opener: Hati-hati, jangan sampai…
Use address terms: Pak/Bu, jangan sampai…

Are there more casual alternatives?

Yes:

Jangan salah belok di depan zebra cross. (Everyday term for crosswalk)
Jangan sampai salah belok sebelum nyebrang. (colloquial nyebrang for menyeberang) Dropping sampai makes it more direct; keeping it sounds like friendly caution.

If I mean “right at the crosswalk,” how do I say that clearly?

Use:

tepat di penyeberangan or pas di penyeberangan (colloquial) = right at the crossing
For “before reaching it,” use sebelum penyeberangan; for “after passing it,” use sesudah/setelah penyeberangan.