"The language of our thoughts and our emotions is our most valuable asset. Multilingualism is our ally in ensuring quality education for all, in promoting inclusion and in combating discrimination. "Irina Bokova, Director-General of UNESCO
From the UNESCO website:
International Mother Language Day has been observed every year since February 2000 to promote linguistic and cultural diversity and multilingualism. The date represents the day in 1952 when students demonstrating for recognition of their language, Bangla, as one of the two national languages of the then Pakistan, were shot and killed by police in Dhaka, the capital of what is now Bangladesh.
Here is a great post about it from Multilingual living.