Completing spanish language (General)
In general you have to keep in mind, that the translations are typically made by several people over the time. As I know from the lessons in spanish language a friend of mine took years ago, the Spanish language can be very diverse, depending of the origin of the speaker and, in case of learning the language, your teacher (Spain, Central America or South America). So it is possible, that parts of the language definitions was translated by (a) Spanish user(s) and other parts by Argentinians or Mexicans (or …). So it takes no wonder, when it is a non-uniform conglomerate.
This is the reason because I don't want to deal with this language-region issues. I only wanted to notice the non-uniform look of this language file, in case the maintainers don't speak spanish.
The first step would be to complete the translation, where possible (your attemt looks very good in this regard). Afterwards feel free to open a pull request on Github so we would be able to insert your changes into the codebase. That would be very nice.
I have finally revised the spanish translation and I think it's ready for the PR. I have added additional commits with latest changes, to see them more easily (sorry, I don't know what happened with this lines: https://github.com/vihuna/mylittleforum/commit/bb9aee9d41596ef1615bdf73cfb8b069593a05c1). If you prefer, I can squash the commits before the PR.
If you want to correct existing translated snippets in a second step, it would be really cool to know your origin (at least the continent or better the country).
I'm from Spain.