I have been trying to improve my writing skills. Most of what I write is in English. To what extent do you think that writing skills are cross-language? I am concerned about my time investment into writing if I move to a Spanish-speaking country.
In my experience, this totally depends on the type of writing.
For technical/professional writing, you can get to a high level relatively easy. Also, in this case your audience should be generally forgiving even if they see that you are writing not in your first language.
For general essays/articles, it would require some practice before your pieces will blend in with native speakers' work. I recommend having a number of native "beta readers" who can point to issues.
For fiction writing... this is something that I consider a "Crowning achievement" for a non-native author. Your work can be very good in terms of plot and ideas, but it may take years of practice to stand on equal footing with the native writers.
Of course all the above applies to a monolingual author who learns other language later in life. For someone who is at least somewhat bilingual from an early age, shifting between languages should be smoother.
Excellent question. I don't think it would be wasted effort at all to practice writing well in English. There are some things that can be transferred over languages, but some things cannot be so easily transferred. It is good practice to write in either language, and the things you can learn will never be wasted.
What can be transferred easiest are your core ideas that make up your story--plot, character development, and other things. These are universal concepts that can be moved easiest from language to language.
What will be more difficult to transfer is your writing style, but assuming that you are fluent in both English and Spanish, this probably won't be as difficult.
I would recommend you practice with both English and Spanish. Don't worry about losing investments of time. Practice is something that is never a waste.