The only other place where I could find languages that did the same was in Central Africa, and my dissertation cited a 1976 article by the great French linguist Claude Hagège which mentioned by name two Nilo-Saharan languages, Moru and Mangbetu, and two Niger-Congo languages, Mbum and M'baka. Over the years, I lost track of anything pertaining to those languages except their names.
But I got curious again recently as I worked on updating for publication an old paper on clause-bracketing in PNG Austronesian languages. So, yesterday, after googling those names and finding out that Mbum and M'baka (= Ngbaka) are spoken in the Central African Republic, I emailed my historian brother in Strasbourg, whom I recently visited, to ask whether he knew of any CAR languages that bracketed their relative clauses. He had spent years working in the (at that time) Central African Empire for the US Peace Corps and USAID while I was writing my dissertation in linguistics, and he later wrote a dissertation himself on Japan-Africa relations before World War II.
My query didn't ring any bells with him at first, but after some reflection he came up with some examples in Sango, CAR's national lingua franca. And then he emailed to ask his linguist friend Raymond Boyd at CNRS whether he could think of Adamawa-Ubangi languages that used such markers for relative clauses. Boyd replied:
Right off, I can't think of one that DOESN'T. In languages like Sango and Chamba, opener and closer can be the same. In Zande, the opener is etymologically an indefinite and the closer is a locative. I've been reading a dissertation on Mambay (an Adamawa language closely related to Mbum and Mundang) where there is only an opener, but I take this to be perhaps a Chadic influence (I'd have to check this on a much larger range of data).It was a Eureka moment for both of us.
I can't believe I never thought to ask my own brother before! Back in January, when he took us to the used book vendors in place Gutenberg in Strasbourg, I discovered a book I couldn't resist buying—despite the 30€ price—for no other reason than that I had mentioned the language it described in my dissertation. It was La Langue des Makere, des Medje et des Mangbetu, par A. Vekens, Dominicain (Editions Dominicaines Veritas, 1928), and the pages were still uncut. But even then, it didn't cross my mind to quiz my brother about the CAR languages he had worked on.
Here are some examples of bracketed relative clauses.
Mangbetu (Vekens 1928) in Congo
A belu [si kesia né môlô ta kira ne] kambuba e faranga môkôtu.
Les hommes [ceux font le travail avec intelligence ceux-là] gagneront des francs beaucoup.
‘Those who work smart get plenty money.’
Sango (my brother, pers. comm.) in CAR
Tene [so mo tene so] ake nzoni ape.
word [thus you say thus] is good not
‘What you say is no good.’
Jabêm (Dempwolff 1939) in PNG
Lip [tec aê gawa nec] gêjac mocseŋ teŋ.
trap [Dem I I-set Dem] it-catch bushfowl one
‘The trap I set caught a bushfowl.’
South Watut (Holzknecht 1989) in PNG
Jek i-ra jiyaʔ ri naip a [ti ra-gin afu ŋga]
Jack he-cut tree with knife [Dem I-give to Dem]
‘Jack cut the tree with the knife which I gave him.’
Patep (Lauck 1980) in PNG
Ông ob tyoo yii yuu nuhu [wê ob lam ge]
you will dodge spear two arrow [Rel will come Rel]
‘You will dodge the spears and arrows that will come.’