Both was (with subordinate clause) and das (in a separate sentence) work:

Meine Großeltern starben beide im Alter von 80 Jahren, was für Indonesier schon ziemlich viel ist.
Meine Großeltern starben beide im Alter von 80 Jahren. Das ist für Indonesier schon ziemlich viel.

The 2nd is the direct analogue to English

My grandparents both died at the age of 80. That is already a pretty good deal for Indonesians.

Tentative answer why welches doesn't work as well as was here, based mostly on oddity to the native ear: I tend to think that welches would be too narrow in that it refers to Alter only, whereas was (or das) refers to the whole situation. (?)

If you look at the example section for which in wiktionary, the which here is like example no. 3 for the relative which, which would also be translated as was in German. (= was im Deutschen auch mit was übersetzt [werden] würde.)

He walked by a door with a sign, which read: PRIVATE OFFICE.
-> welches/das; ignoring that signs don't read in German

A situation in which tensions are high.
-> in welcher/in der

He had to leave, which was very difficult.
-> was/das + taking the sentence apart

No art can be properly understood apart from the culture of which it is a part.
-> derer/[von welcher]; rather different construction

An alternative that works both in German and English is that you can make two sentences with that/das:

My grandparents both died at the age of 80. That is already a is already a pretty good deal for Indonesians.
Meine beiden Großeltern starben im Alter von 80 Jahren. Das ist für Indonesier schon ziemlich viel.

I tend to think that whenever you can use that for the which in English*, German uses was within one sentence or das in a two sentence construction. Which I'd think covers the whole "which is"-ing. The construction with ", was" is very common in German, so in that sense I'd say, yes, "which is"-ing is done in German very much like in English - only it translates to was and not like many other whiches to welche/r/s.

* To me, trying what for this type of which sounds unusual but not entirely impossible. This may be an old link between English and German - or my lovely German accent in English grammar ;-)

Both was (with subordinate clause) and das (in a separate sentence) work:

Meine Großeltern starben beide im Alter von 80 Jahren, was für Indonesier schon ziemlich viel ist.
Meine Großeltern starben beide im Alter von 80 Jahren. Das ist für Indonesier schon ziemlich viel.

The 2nd is the direct analogue to English

My grandparents both died at the age of 80. That is already a pretty good deal for Indonesians.

Tentative answer why welches (nor das in a subordinate clause construction) doesn't work as well as was here, based mostly on oddity to the native ear: I tend to think that welches would be too narrow in that it refers to Alter only, whereas was (or das) refers to the whole situation. (?)

If you look at the example section for which in wiktionary, the which here is like example no. 3 for the relative which, which would also be translated as was in German. (= was im Deutschen auch mit was übersetzt [werden] würde.)

He walked by a door with a sign, which read: PRIVATE OFFICE.
-> welches/das; ignoring that signs don't read in German

A situation in which tensions are high.
-> in welcher/in der

He had to leave, which was very difficult.
-> was/das + taking the sentence apart

No art can be properly understood apart from the culture of which it is a part.
-> derer/[von welcher]; rather different construction

An alternative that works both in German and English is that you can make two sentences with that/das:

My grandparents both died at the age of 80. That is already a is already a pretty good deal for Indonesians.
Meine beiden Großeltern starben im Alter von 80 Jahren. Das ist für Indonesier schon ziemlich viel.

I tend to think that this covers the whole "which is"-ing. The construction with ", was" is very common in German, so in that sense I'd say, yes, "which is"-ing is done in German very much like in English - only it translates to was and not like many other whiches to welche/r/s.

* To me, trying what for this type of which sounds unusual but not entirely impossible. This may be an old link between English and German - or my lovely German accent in English grammar ;-)

Both was (with subordinate clause) and das (in a separate sentence) work:

Meine Großeltern starben beide im Alter von 80 Jahren, was für Indonesier schon ziemlich viel ist.
Meine Großeltern starben beide im Alter von 80 Jahren. Das ist für Indonesier schon ziemlich viel.

The 2nd is the direct analogue to English

My grandparents both died at the age of 80. That is already a pretty good deal for Indonesians.

Tentative answer why welches doesn't work as well as was here, based mostly on oddity to the native ear: I tend to think that welches would be too narrow in that it refers to Alter only, whereas was (or das) refers to the whole situation. (?)

If you look at the example section for which in wiktionary, the which here is like example no. 3 for the relative which, which would also be translated as was in German. (= was im Deutschen auch mit was übersetzt [werden] würde.)

He walked by a door with a sign, which read: PRIVATE OFFICE.
-> welches/das; ignoring that signs don't read in German

A situation in which tensions are high.
-> in welcher/in der

He had to leave, which was very difficult.
-> was/das + taking the sentence apart

No art can be properly understood apart from the culture of which it is a part.
-> derer/[von welcher]; rather different construction

An alternative that works both in German and English is that you can make two sentences with that/das:

My grandparents both died at the age of 80. That is already a is already a pretty good deal for Indonesians.
Meine beiden Großeltern starben im Alter von 80 Jahren. Das ist für Indonesier schon ziemlich viel.

I tend to think that whenever you can use that for the which in English*, German uses was within one sentence or das in a two sentence construction. Which I'd think covers the whole "which is"-ing.

* To me, trying what for this type of which sounds unusual but not entirely impossible. This may be an old link between English and German - or my lovely German accent in English grammar ;-)

Both was (with subordinate clause) and das (in a separate sentence) work:

Meine Großeltern starben beide im Alter von 80 Jahren, was für Indonesier schon ziemlich viel ist.
Meine Großeltern starben beide im Alter von 80 Jahren. Das ist für Indonesier schon ziemlich viel.

The 2nd is the direct analogue to English

My grandparents both died at the age of 80. That is already a pretty good deal for Indonesians.

Tentative answer why welches doesn't work as well as was here, based mostly on oddity to the native ear: I tend to think that welches would be too narrow in that it refers to Alter only, whereas was (or das) refers to the whole situation. (?)

If you look at the example section for which in wiktionary, the which here is like example no. 3 for the relative which, which would also be translated as was in German. (= was im Deutschen auch mit was übersetzt [werden] würde.)

He walked by a door with a sign, which read: PRIVATE OFFICE.
-> welches/das; ignoring that signs don't read in German

A situation in which tensions are high.
-> in welcher/in der

He had to leave, which was very difficult.
-> was/das + taking the sentence apart

No art can be properly understood apart from the culture of which it is a part.
-> derer/[von welcher]; rather different construction

An alternative that works both in German and English is that you can make two sentences with that/das:

My grandparents both died at the age of 80. That is already a is already a pretty good deal for Indonesians.
Meine beiden Großeltern starben im Alter von 80 Jahren. Das ist für Indonesier schon ziemlich viel.

I tend to think that whenever you can use that for the which in English*, German uses was within one sentence or das in a two sentence construction. Which I'd think covers the whole "which is"-ing. The construction with ", was" is very common in German, so in that sense I'd say, yes, "which is"-ing is done in German very much like in English - only it translates to was and not like many other whiches to welche/r/s.

* To me, trying what for this type of which sounds unusual but not entirely impossible. This may be an old link between English and German - or my lovely German accent in English grammar ;-)