What Takasaki translates
as the Ratnadārikā-sūtra is in the parallel Chinese the 寶女經 Bǎonǚ jīng, which I presume is the 寶女所問經 Bǎonǚ suǒ wèn jīng, Taisho 399. Here is the Chinese text
, it is four fascicles long.
According to The Korean Buddhist Canon: A Descriptive Catalogue by Dr. Lewis R. Lancaster, 1979
, the Sanskrit name of Taisho 399 is "Mahāyānopadeśa(sūtra)" and the Tibetan name is "Theg-pa chen-poḥi man-nag".
The multilingual equivalents in Lancaster's 1979 catalogue are sometimes out of date
, but at least it gives you some new leads in your search.
There is no translation of Taisho 399 listed on Marcus Bingenheimer's Bibliography of Translations from the Chinese Buddhist Canon into Western Languages
Taisho 399 is indeed equivalent to chapter 3 (寶女品) of Taisho 397 大方等大集經 (Mahāvaipulyamahā)sannipātasūtra
. Here is the Chinese
(chapter 3 starts in fascicle 5). There are no translations of this listed in Bingenheimer's Bibliography either.