Traditionally, character's under Unix were encoded in a locale-specific manner: ISO 8859-1 in Western Europe, ISO 8859-2 in Eastern Europe, EUC-JP in Japan, etc. In the 1990s, there was a major push to get XFree86 (the ancestor of X.Org) to switch to locale-independent UTF-8, lead mainly by Markus Kuhn and Bruno Haible.

The link is to Markus Kuhn's web page, which appears to describe the UTF_8 software available around 1998 or so.