The SEI maintains:
The Unicode CLDR Project maintains a list of languages and scripts.
See also Anshuman Pandey's Expanding Unicode and
Everson's Papers Formally Submitted to the UTC and ISO/IEC 10646 JTC 1/SC2/WG2, for more recently revised proposals.
For information on modern languages, see the Ethnologue entries, and follow the links for the "Search the Web" version.
For further information on historic and constructed languages (with links to the modern languages in the Ethnologue) see the LinguistList website.
The Roadmap contains the official list of scripts needing encoding. The Roadmap organizes the scripts in different planes, with proportional maps of actual and proposed allocations to Unicode (ISO/IEC 10646): most modern scripts are found in the Basic Multilingual Plane (BMP, or Plane 0). Many archaic scripts are located in the Supplementary Multilingual Plane (SMP, or Plane 1).
On the Roadmap, the scripts listed with question marks surrounding them in red identify those scripts that currently lack a detailed proposal. The names with green also need further work, i.e., expert review and additional research. Clicking on any underlined name will take you to the proposal. Some scripts may still be missing from the Roadmap.
Input is needed from specialists on any outstanding scripts. This can be in the form of (a) comments on outstanding script proposals, (b) information in support of an ongoing proposal, (c) letters in support of a specific proposal, or (d) actual writing of a proposal.
If you would like to write a letter of support for a specific proposal, please email us.
(c) If you would like to write a proposal, please review the Unicode guidelines, look through some of the proposals found on Michael Everson's "Papers Formally Submitted to the UTC and ISO/IEC 10646 JTC 1/SC2/WG2" at http://www.evertype.com/formal.html or those on Anshuman Pandey's "Digital Standards for South Asian Writing Systems" at http://linguistics.berkeley.edu/~pandey/, then send an email to Deborah Anderson (email@example.com).
If you are new to Unicode, it is advisable to read the beginning chapters of the Unicode Standard (available online on the Unicode website, http://www.unicode.org/standard/standard.html). If you intend to become involved in writing proposals, be sure to review the Unicode Consortium Policies, and Proposal Guidelines.