Bab.la is an excellent online language resource with online translation as one of its core services — in an impressive 22 languages.
The site makes heavy use of flags as iconography, with some interesting flag choices: Arabic is represented by the Egyptian flag, Swahili by the Kenyan flag and Hindi by the Indian flag.
Arabic is an official language in 24 countries other than Egypt (however, Egypt is by far the largest by population) and Swahili is an official language in four countries. While Hindi is the most widely spoken language in India, there are 22 official languages in India.
Furthermore, English is represented by the Union Jack; however it is an official language in over 50 countries. Also, the United States which has many more English-speakers than the United Kingdom. (The same argument can be applied to the use of Spanish and Portuguese flags for those languages in relation to Mexico and Brazil respectively).
Regardless of whether flags are appropriate in this situation, another question can be posed: do the flags actually aid in legibility of the translation menu? Or are they actually distracting?
Consider the Google Translate interface that supports 65 languages:
Arguably the Google Translate is easier to read and far more compact on screen: and it achieves this in part by not using flags to represent languages.