It seems, on the other hand, that the author herself has included in the original version of the novel in English a series of terms that have been kept in the original Nigerian language (or one of its dialects) and that have been explained by the author herself in footnotes or other references. This is an interesting point that may serve as the focus of the theoretical discussion between foreignization and domestication and that applies to your translation and commentary:
Should the translator keep the original terms in Nigerian in the translation?
Should the translator provide footnotes (as the author does) to explain such terms?
Should the translator forget about the terms in Nigerian and translated them directly to Spanish in the novel and dispense with the footnotes?
Here you have an interesting debate in which you could focus the general debate on foreignization and domestication. As I say, is a very global and complex debate, which you could perhaps define more closely by focusing on the previous: translation strategies regarding the inclusion of original terms in the author’s mother tongue, within a novel written in another language. If your view is that the translation should keep the terms in Nigerian and provide appropriate footnotes, as the author does, then you’ll need to back such idea with appropriate arguments (to be found in your literature review). If your view is that the terms in Nigerian should be kept but no explanation should be offered in footnotes (a more extreme foreignization, perhaps), then, you’ll need to provide arguments for this too. Whatever is your translation strategy, you’ll need to include it within the general debate of foreignization/domestication on that specific issue and be able to contribute to that debate.
As you say, other authors like Ishiguro does not seem to have this problem because they want to make their books as translatable as possible. In the case of Okparanta, this seems to be different, and provide an interesting justification for your translation and remit.