As it seems there's no native way to implement global translations (i.e., shared translations that can be used in different templates/modules), let me share the solution I have implemented, which I have described in my blog:
http://jordisan.net/blog/2016/global-translations-in-prestashop/
In short, I'm using a new .tpl file for global translations which can be included from different modules and themes/templates; the string to translate is passed to that template as a variable. Using this solution, these global translations can be managed through PrestaShop's back office.
Any feedback will be welcome!