As I suggested in some other thread: An official language of the country has to be used at least once per four years. A lot of countries already meet that requirement, but it would force a select few countries to send something in their own language/languages once in awhile. For example, I think it's pretty sad that the only song in Swedish since 1998 (the last time Sweden was forced to) was a Finnish entry.
Another thing I have previously suggested is using subtitles for the songs in all countries. This is done in a few countries, and I believe it would lower the threshold for sending a song in a language most people don't understand. Understanding the lyrics is very important for a lot of people.