I have my Mac OS X Yosemite system configured in spanish, but I have an app with a horrible spanish translation, and I would like to run it (and just this app) in english. How could I do that?
You may try to move translation files out of application bundle. First make a copy of Your app (just in case). Then go to the Resources folder inside this app-copy (right click -> show package content -> enter Content -> enter Resources). Here You'll probably find directory called like: es_ES.lproj/es_lproj/es_MX.lproj (or similar) - delete this dir. Launch this app-copy and check the language.
This is a bit overkill since You can use
-AppleLanguages switch. Good answers in: Is it possible to launch a program in a specific language?