I just start reading PTS's big three-volume translation of the Jatakas. 95% of the time, PTS is honest about their titles. If a book is part of the Tipitika, then the translation is only a translation of the Tipitika, uncorrupted by commentary. Certainly the translators refer to the commentaries when making their translations, sometimes overriding them, and sometimes there's a little bit of commentary in the footnotes, which is great, but there's no way anyone could mistake commentary for Tipitika.
Reaching the Khuddhaka, I was surprised to discover that the PTS Vimaanavatthu translation was not just Tipitika, but had commentary mixed in. When I started reading the PTS Jataka translation, and got to the eighth sutta and it seemed to be speaking in the translator's voice, I became suspicious and got out my trusty DPR and came to the startling conclusion that the Jatakas are only a few verses each, having nothing to do with previous incarnations. The fat three-volume set is mostly commentary. A translation of the Jatakas would be a single slim volume.
This was startling to me because I had always heard that the Jatakas are about previous incarnations of the bodhisatta, and now I realize that that's just commentary. The Jatakas themselves are just a few verses each, similar to the Dhammapada, etc. Nina suggested that the Jataka verses might be older, but I hadn't realized that only the verses were in the Tipitika.
I hope that one day a translation of the Jatakas free of commentary is produced. For now I will just read the verses and skip over the commentary.
If I'm mistaken about this that only the verses are in the Tipitika, then correct me, but I've double-checked on Metta.lk and the Jatakas appear to be very small and only the verses.