I've just recently learned about .po and .mo files in WordPress Themes. My question is, are these files supposed to be used to translate page and post content or just the WordPress backend along with Plugins?
.po and .mo files are for translating strings in code files. For translating post content use a translation plugin. I've used a few and find WPML to be the best in many aspects.
Incidentally, they have a built-in workflow for translating both content and strings in code files.