Now that there are two different Spanish translations, the original international one and the traditional one (Spanish-es), TW does not recognize the file spanish-es.lng. I suspect it doesn't like the hyphen. The only way to make it work is to change the name to spanish.lng (which is the name of the other spanish file) or something like spanish_es.lng.
It seems to me that the spanish-es.lng.exe will have to be fixed and the name of the installed file changed, or else TW will need to be tweaked in order to accept hyphens in file names.
For translators. Undertake the job to translate to your language, report translation errors here. Co-ordinate with other working on a translation of the program.