> I am looking for examples of English texts written by Spanish-speaking
> learners of English
María Belén Díez Bedmar reviewed the available corpora a few years ago: Diez-Bedmar, M. B. (2009). Written learner corpora by Spanish students of English: an overview. Paper presented to the Asociación Española de Lingüística del Corpus 2009 conference. Murcia: Asociación Española de Lingüística del Corpus. Available from http://www.um.es/lacell/aelinco/contenido/pdf/62.pdf
There are probably some new resources that have been made publicly available since, but I don't follow the field closely. Have you checked the ‘Learner corpora around the world’ page at the Catholic University of Louvain? https://www.uclouvain.be/en-cecl-lcworld.html
HTH,
Alon