Should I use country flags in language selection fields?

My users can create content in different languages, and I need to provide an option to choose the language when they enter new content.

Right now there are only 5 languages (en, fr, de, pt, es), and I was wondering if using country flags is the correct way to represent languages, or should I rather just provide labels (English, French, Deutsch, ...), since I'm never really sure if I should use the American or British flag to represent the English language.

Right now, the system is intended to store technical information, so there wouldn't be a need for en-gb or en-us or other sub-languages.