The day was marked at Unesco headquarters in Paris by a conference on linguistic diversity focusing on the difficulties of African, Asian and American minorities to preserve their traditional tongues.
Koichiro Matsuura, Unesco director-general, said: "When a language dies, it is a vision of the world that disappears.
"Language is much more than an instrument, considerably more than a tool."
He added: "In structuring our thoughts, in coordinating our social relations and in building our relationship with reality, it constitutes a fundamental dimension of the human being."
Internet a factor
A major part of Unesco's efforts to safeguard languages is aimed at ensuring greater diversity on the internet and in official texts, the organisation said.
Today 72% of internet sites are in English, followed by German at just 7%, and French, Japanese and Spanish at 3%.
Matsuura: When a language dies,
a vision of the world disappears
About 90% of the world's languages are not represented at all on the internet, Unesco said.
Some 20% of languages have no written version.
In Africa - where one-third of the world's languages are spoken - about 80% of these are purely oral, and thus in greater danger of dying out, Unesco said.
The African Union has declared 2006 a year of African Languages.