You don't gain the needed rights to declare something as Product identity by merely translating
When translating something, you become the author of a derivate of a work, but not the sole author. As you are not the sole author, the joint copyright in the derivate rests within the original author and you - but you are still bound by the OGL. You are not the sole author, and you can't declare something Product Identity unless all other contributors agree. And you can't undeclare something that had been OGC to become Product Identity - the license granted to the content before your translation is perpetual, and you can not revoke someone else's license granted by a different party (the original author).
You only got the license to make your derivate by the OGL 1.0a. The OGL says, that a derivate work, especially translations, needs to be under OGL license in paragraph 2. Since your translation is a derivate work, not a free-standing work or your own product identity, you need to release your translation under the OGL. Your translation is of OGC, and you can't make it Product Identity yourself, and in fact, you may not even use any Product identity in your translation! To use the Product Identity of someone, you need to license that material differently - and any and all names and characters automatically are in this category. So you can't just translate someone's adventure unless you gain a release.
Everything else you take from the OGC you have to release under OGL. If it was OGC, it has to stay OGC. If you make it yourself, you can choose to make it Product Identity or OGC. If it was Product Identity, you can't touch it without a separate license. The only person that can re-declare something Product Identity (or rather: OGC) is the original author or any subsequent rights owner (as they are the only ones who can declare something OGC under section 5). They had chosen OGC.
You can't change that choice, as you don't own all the needed rights. You own only the expression of the translation itself, but you are by definition a co-author of the translated work. You and the original author both hold claim to the exact same parts of the resulting work: the text expression. But that is different from a transformative work, where you hold sole copyright in any additions - which is what allows for example Paizo to declare items Product Identity: their additions are product identity. To declare a mere translation Product Identity, you need to obtain a different license from the original author than OGL.
The relevant parts of the OGL 1.0a are (emphasis mine):
(a)"Contributors" means the copyright and/or trademark owners who have contributed Open Game Content;
(b)"Derivative Material" means copyrighted material including derivative works and translations (including into other computer languages), potation, modification, correction, addition, extension, upgrade, improvement, compilation, abridgment or other form in which an existing work may be recast, transformed or adapted;
(c) "Distribute" means to reproduce, license, rent, lease, sell, broadcast, publicly display, transmit or otherwise distribute;
(d)"Open Game Content" means the game mechanic and includes the methods, procedures, processes and routines to the extent such content does not embody the Product Identity and is an enhancement over the prior art and any additional content clearly identified as Open Game Content by the Contributor, and means any work covered by this License, including translations and derivative works under copyright law, but specifically excludes Product Identity.
(e) "Product Identity" means product and product line names, logos and identifying marks including trade dress; artifacts; creatures characters; stories, storylines, plots, thematic elements, dialogue, incidents, language, artwork, symbols, designs, depictions, likenesses, formats, poses, concepts, themes and graphic, photographic and other visual or audio representations; names and descriptions of characters, spells, enchantments, personalities, teams, personas, likenesses and special abilities; places, locations, environments, creatures, equipment, magical or supernatural abilities or effects, logos, symbols, or graphic designs; and any other trademark or registered trademark clearly identified as Product identity by the owner of the Product Identity, and which specifically excludes the Open Game Content;
(f) "Trademark" means the logos, names, mark, sign, motto, designs that are used by a Contributor to identify itself or its products or the associated products contributed to the Open Game License by the Contributor
(g) "Use", "Used" or "Using" means to use, Distribute, copy, edit, format, modify, translate and otherwise create Derivative Material of Open Game Content.
- The License:
This License applies to any Open Game Content that contains a notice indicating that the Open Game Content may only be Used under and in terms of this License. You must affix such a notice to any Open Game Content that you Use. No terms may be added to or subtracted from this License except as described by the License itself. No other terms or conditions may be applied to any Open Game Content distributed using this License.
- Grant and Consideration:
In consideration for agreeing to use this License, the Contributors grant You a perpetual, worldwide, royalty-free, non-exclusive license with the exact terms of this License to Use, the Open Game Content.
5.Representation of Authority to Contribute:
If You are contributing original material as Open Game Content, You represent that Your Contributions are Your original creation and/or You have sufficient rights to grant the rights conveyed by this License.
- Use of Product Identity:
You agree not to Use any Product Identity, including as an indication as to compatibility, except as expressly licensed in another, independent Agreement with the owner of each element of that Product Identity.