Return to Answer

1a. [bumbu Jepang -> MISO] + [jual murah -> OBRAL] minus [pakaian dalam -> BRA]; Misool is one of the Raja Anpat islands.
5a. [alam baka -> AKHIRAT] minus [di dalam Bahasa Inggris -> AT, perhaps because "at" is an English translation of "di dalam"?]; def is "ujung".
8a. [janji -> IKRAT] minus [Amien Rais -> AR]; def is "sudah kubilang kan, disingkat di Inggris" -> "I know, right?" -> IKR.
10a. [ketika -> TEMPO] with [donat -> O] replaced by [nilai terendah -> E, lowest grade in exams]; def is "makanan dari kedelai".
11a. [salah satu jajanan tradisional -> LEMPER] minus [pegas -> PER]; def is "pelengket".
12a. Double def, or maybe single cryptic def.
13a. [permaianan kata Bahasa Inggris -> PUN]; def is "demikian".
15a. [sedia -> SIAP] before [nilai tertinggi -> A, highest grade in exams]; def is "seseorang". Not sure what's up with "belum tentu", which seems to reverse the meaning in a way that isn't called for; probably this just indicates that I don't understand the language.
18a. [noda hitam -> TOMPEL] minus TOM; def is "kain pembersih".
19a. [dengki -> SIRIK] in reverse; def is "sebanga pohon kaku".
20a. Double def; "warna nila" is the colour, "memiliki kemapuan supranatural" is a reference to the New Age-y concept of "indigo child".

1d. [marah -> MURKA] minus [kereta api -> KA; I think this is a standard Indonesian abbreviation]. I confess I don't know why pelat logam (lit. "metal plates") -> MUR (which I think means wall).
2d. [lingkungan kehidupan -> EKOSISTEM] minus [metode -> SISTEM], reversed. Def is "setuju".
3d. [pacuan -> BALAP] minus [plat nomor Sumatera Barat -> BA]; def is "penggosok".
4d. [Terjemahkan ke Bahasa Inggris: bibir -> LIP]< = PIL. Def is "tablet".
6d. KETIAK*; def is "Saat".
7d. METEOR*; def is "pengendali jarak jauh".
9d. [hewan gurun -> UNTA] anagrammed to ANTU inside [karta keluagra -> KK, standard Indonesian abbreviation]; def is "menguap" or maybe "[yang] membuat menguap". This uses an indirect anagram, which is a bit naughty.
10d. [saat -> ERA] inside [Taman Safari Indonesia -> TSI, which I assume is an obvious thing to abbreviate by its initials]; def is "bumbu sambal".
14d. Double def; NIL is both nothing (kosong) and the river Nile (salah satu nama sungai).
16d. Def is "inti"; wordplay remains a mystery.
17d. First part of clue means "doctoral study abroad"; second half has initials PHD. (I didn't see anything actually indicating that initials should be taken, though.)
18d. [tidak setuju-> PROTES] minus [ujian -> TES]; def is "setuju".

For many of these there are more verbose, but also more disorganized, explanations in the chatroom linked above. (In particular, there are lots of Bahasa Indonesia -> English translations there.)

1a. [bumbu Jepang -> MISO] + [jual murah -> OBRAL] minus [pakaian dalam -> BRA]; Misool is one of the Raja Anpat islands.
5a. [alam baka -> AKHIRAT] minus [di dalam Bahasa Inggris -> AT, because "at" is one English translation of "di"]; def is "ujung".
8a. [janji -> IKRAT] minus [Amien Rais -> AR]; def is "sudah kubilang kan, disingkat di Inggris" -> "I know, right?" -> IKR.
10a. [ketika -> TEMPO] with [donat -> O] replaced by [nilai terendah -> E, lowest grade in exams]; def is "makanan dari kedelai".
11a. [salah satu jajanan tradisional -> LEMPER] minus [pegas -> PER]; def is "pelengket".
12a. Double def, or maybe single cryptic def.
13a. [permaianan kata Bahasa Inggris -> PUN]; def is "demikian".
15a. [sedia -> SIAP] before [nilai tertinggi -> A, highest grade in exams]; def is "seseorang yang belum tentu".
18a. [noda hitam -> TOMPEL] minus TOM; def is "kain pembersih".
19a. [dengki -> SIRIK] in reverse; def is "sebanga pohon kaku".
20a. Double def; "warna nila" is the colour, "memiliki kemapuan supranatural" is a reference to the New Age-y concept of "indigo child".

1d. [marah -> MURKA] minus [kereta api -> KA; I think this is a standard Indonesian abbreviation]; def is "pelat logam"; OP clarifies that a mur is a nut (as in "nuts and bolts"); I'm still a little puzzled how "metal plate" clues that, but no matter :-).
2d. [lingkungan kehidupan -> EKOSISTEM] minus [metode -> SISTEM], reversed. Def is "setuju".
3d. [pacuan -> BALAP] minus [plat nomor Sumatera Barat -> BA]; def is "penggosok".
4d. [Terjemahkan ke Bahasa Inggris: bibir -> LIP]< = PIL. Def is "tablet".
6d. KETIAK*; def is "Saat".
7d. METEOR*; def is "pengendali jarak jauh".
9d. [hewan gurun -> UNTA] anagrammed to ANTU inside [karta keluagra -> KK, standard Indonesian abbreviation]; def is "menguap" or maybe "[yang] membuat menguap". This uses an indirect anagram, which is a bit naughty.
10d. [saat -> ERA] inside [Taman Safari Indonesia -> TSI, which I assume is an obvious thing to abbreviate by its initials]; def is "bumbu sambal".
14d. Double def; NIL is both nothing (kosong) and the river Nile (salah satu nama sungai).
16d. [celah seperti pada pintu -> KISI] without its head; def is "inti".
17d. First part of clue means "doctoral study abroad"; second half has initials PHD. (Apparently this is a standard abbreviation in Indonesia.)
18d. [tidak setuju-> PROTES] minus [ujian -> TES]; def is "setuju".

For many of these there are more verbose, but also more disorganized, explanations in the chatroom linked above. (In particular, there are lots of Bahasa Indonesia -> English translations there.)

Thanks to athin for clarifying in comments some bits of wordplay that we missed.

You can find explanations of more or less everything there in the chatroom, though there are a few clues where we were confident of the answer but in the dark about the wordplay.

Wordplay explanations -- across clues:

1a. [bumbu Jepang -> MISO] + [jual murah -> OBRAL] minus [pakaian dalam -> BRA]; Misool is one of the Raja Anpat islands.
5a. [alam baka -> AKHIRAT] minus [di dalam Bahasa Inggris -> AT, perhaps because "at" is an English translation of "di dalam"?]; def is "ujung".
8a. [janji -> IKRAT] minus [Amien Rais -> AR]; def is "sudah kubilang kan, disingkat di Inggris" -> "I know, right?" -> IKR.
10a. [ketika -> TEMPO] with [donat -> O] replaced by [nilai terendah -> E, lowest grade in exams]; def is "makanan dari kedelai".
11a. [salah satu jajanan tradisional -> LEMPER] minus [pegas -> PER]; def is "pelengket".
12a. Double def, or maybe single cryptic def.
13a. [permaianan kata Bahasa Inggris -> PUN]; def is "demikian".
15a. [sedia -> SIAP] before [nilai tertinggi -> A, highest grade in exams]; def is "seseorang". Not sure what's up with "belum tentu", which seems to reverse the meaning in a way that isn't called for; probably this just indicates that I don't understand the language.
18a. [noda hitam -> TOMPEL] minus TOM; def is "kain pembersih".
19a. [dengki -> SIRIK] in reverse; def is "sebanga pohon kaku".
20a. Double def; "warna nila" is the colour, "memiliki kemapuan supranatural" is a reference to the New Age-y concept of "indigo child".

and down clues:

1d. [marah -> MURKA] minus [kereta api -> KA; I think this is a standard Indonesian abbreviation]. I confess I don't know why pelat logam (lit. "metal plates") -> MUR (which I think means wall).
2d. [lingkungan kehidupan -> EKOSISTEM] minus [metode -> SISTEM], reversed. Def is "setuju".
3d. [pacuan -> BALAP] minus [plat nomor Sumatera Barat -> BA]; def is "penggosok".
4d. [Terjemahkan ke Bahasa Inggris: bibir -> LIP]< = PIL. Def is "tablet".
6d. KETIAK*; def is "Saat".
7d. METEOR*; def is "pengendali jarak jauh".
9d. [hewan gurun -> UNTA] anagrammed to ANTU inside [karta keluagra -> KK, standard Indonesian abbreviation]; def is "menguap" or maybe "[yang] membuat menguap". This uses an indirect anagram, which is a bit naughty.
10d. [saat -> ERA] inside [Taman Safari Indonesia -> TSI, which I assume is an obvious thing to abbreviate by its initials]; def is "bumbu sambal".
14d. Double def; NIL is both nothing (kosong) and the river Nile (salah satu nama sungai).
16d. Def is "inti"; wordplay remains a mystery.
17d. First part of clue means "doctoral study abroad"; second half has initials PHD. (I didn't see anything actually indicating that initials should be taken, though.)
18d. [tidak setuju-> PROTES] minus [ujian -> TES]; def is "setuju".

For many of these there are more verbose, but also more disorganized, explanations in the chatroom linked above. (In particular, there are lots of Bahasa Indonesia -> English translations there.)

Annoyingly partial answer

In the chatroom created by North, he and I have been attacking this, and we've nearly done it. Two clues remain unsolved and I fear they may just be too tough. Or maybe inspiration will strike later. For now, I need to go to bed.

Here's the grid so far:

You can find explanations of more or less everything there in the chatroom, though there are a few clues where we were confident of the answer but in the dark about the wordplay.

The two clues that remain unsolved (athin has kindly confirmed that the others are correct) are 18a and 18d. We know (because athin kindly confirmed this too) that these two are both actual Bahasa Indonesia words, and not abbreviated. Here are some disorganized thoughts about them; maybe they'll give someone else ideas, or something.

18a:

Word-by-word translation of the clue is something like: fabric cleaning stain black in/on face/front/advance Tom vanish. Probably the def is "cloth" or "cleaning cloth" and we need to find something that means "black stain" or "cleaning black stain" and then ... remove "tom" or something clued by "Tom" from its front? Or maybe the def is "black stain cleaning cloth" and we start with a word meaning "face" or similar and remove a tom from it. (Much less likely: "pembersih" is a removal indicator somehow; hard to see how the clue fits together then.) But despite some effort we haven't found a word matching -E- and meaning a (cleaning?) cloth, nor anything else that would help with the wordplay. Some words that haven't helped us:
black: hitam erang
stain: noda selekeh pewarna zat_warna conteng renjis calit celekeh
blot: aib cela
Actually, we did find one -E- word with the right sort of meaning: a DEK is a cloth (though not a cleaning cloth). But there don't seem to be any D-O words to go in 18d in that case, and how the wordplay would work is a mystery.

18d:

Word-for-word translation of clue: Agree, not agree; not exist exam!. Letters are --O. This has rather a lot of possible parsings. Most likely ones:
Def "agree". WP: [not agree] minus [exam] or [not agree] minus ADA plus [exam].
Def "exam". WP: [agree] minus [agree] minus [exist] or [agree] minus [not agree].
Only halfway plausible word found so far: EKO meaning "echo", which might be close enough to "agree". No success getting any of the other bits to fit, though, and this would make 18a be EE- which doesn't seem to match any words. Some words that haven't helped so far:
agree: setuju sesuai berkenan menyetujui sepakat mengakui cocok bermufakat mengaku bertemu memadu
concur: bertepatan sependapat sependirian
match: jodoh
affirm: megiakan
correspond: sama
exam: tes uji kir cek; maybe UN (national exams in Indonesia)

Credit where due: the majority of the work here is mine but North solved several of the clues too, including at least 12a, 15a, and 10d, and made a bunch of other suggestions.

Pick a clue. Translate it with Google Translate. Translate individual words and, where appropriate, phrases with Google Translate. Think of possible ways to parse as cryptic clues. Figure out what things we therefore need words for in Bahasa Indonesia, and get 'em with Google Translate again. Repeat until blood starts to pour from every orifice and/or you solve the clue. Repeat for other clues. (You can find some more concrete thinking-out-loud in the chatroom transcript.) In extreme cases, find Indonesian word lists like the ones here and search for words matching the patterns in the grid. (We haven't done this much because it's (1) a bit cheaty and (2) a lot boring.)

In the chatroom created by North, he and I have been attacking this, and we've finally done it.

Here's the grid:

You can find explanations of more or less everything there in the chatroom, though there are a few clues where we were confident of the answer but in the dark about the wordplay.

Pick a clue. Translate it with Google Translate. Translate individual words and, where appropriate, phrases with Google Translate. Think of possible ways to parse as cryptic clues. Figure out what things we therefore need words for in Bahasa Indonesia, and get 'em with Google Translate again. Repeat until blood starts to pour from every orifice and/or you solve the clue. Repeat for other clues. (You can find some more concrete thinking-out-loud in the chatroom transcript.) In extreme cases, find Indonesian word lists like the ones here and search for words matching the patterns in the grid. We haven't done this much because it's (1) a bit cheaty and (2) a lot boring. For the last couple, I made use of a larger wordlist in the same GitHub repository. 18a: looked for words of form -E- and looked them all up in GT, looked for 6-letter words containing TOM, and saw that PEL and TOMPEL were good, with PEL even having the right sort of meaning. Google Translate didn't know TOMPEL (which is why we didn't find it earlier) but the Indonesian Wiktionary did. (Top tip: applying Google Translate to foreign-language Wiktionary pages seems to be quite effective.) With 18a out of the way, 18d was P-O and there weren't a lot of options; again Wiktionary was helpful, actually using the word "agree" to describe PRO (as opposed to KONTRA, when describing votes). Then another search of the bigger wordlist turned up PROTES and I felt very stupid for not thinking of that earlier. (Of course it means "protest", and a TES is a test or exam.)

That was a lot of fun, but also a lot of work.