Translations lost on theme update

I made Spanish and Portuguese language files for my Nirvana theme installation (front-end translations only, no admin). I uploaded them to my nirvana/languages directory and they worked properly. However, as I suspected would happen, they were deleted when I installed the latest theme update. I simply re-uploaded them via FTP, but that’s kind of a pain and also something I can’t ask my client to do once he takes over the site.

Looking at the theme setup code, it appears only the nirvana/languages directory is checked for language files. Is there any way to preserve my custom translations other than to try writing my own theme setup code using my child theme? If not, would it be possible to change this behavior in a future update (i.e., check WP_LANG_DIR/nirvana or nirvana-child/languages before defaulting to nirvana/languages)?