Please don't. Flags are not languages. (http://www.flagsarenotlanguages.com/) There are many languages which cannot be clearly distinguished with a flag icon -- for example, there are dozens of languages which could be represented by an Indian flag (Hindi, Bengali, Telugu, Urdu...), and some languages which don't have any clear flag, or whose flag could be politically complicated to display (like Catalan, Romani, or Tibetan).
Use the localized name of a language to indicate the language (e.g. English, Español, Svenska, 中文). It's unambiguous and takes very little effort to implement.
On the contrary, if you just use the name of the language then it's not at all clear to a user who's using the "wrong" language where the language selector is.
lmm may be suggesting the use of flags as iconography only for the button that opens a popup where you can choose a language. That would be somewhat less bad.
Iconography already exists for that. On Windows, it’s a symbol comprising an A and a 文. On Apple systems, it’s a globe icon. I think the globe icon is seen more often on other platforms and on websites. Having the localised language name next to the icon helps, too.
