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.
However, you only got the license to make your derivate by the OGL 1.0a. The OGL says, that all derivate work, especially translations, need 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 OGL license. Your translation is 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.
Everything else you have to release under OGL. If it was OGC, it has to stay 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 would be the original author, but they had chosen OGC instead. You can’t change that.
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.
- 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.