There are organizations in several countries - here are the ones we know about:

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FINLAND
Föreningen Tvåspråkiga Familjer
c/o Maria Suokangas
Svedjeträdssvängen 5 A 2
023240 Esbo
Finland

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GERMANY

The following information was supplied by Martina:

Information about the ImF (Interessengemeinschaft mehrsprachiger Familien--Interest Group for Multilingual Families in Germany) can be found at the IMF site , which you can read in English or German. We have members all over Germany, although we are still small (but growing fast--around 150 families at the moment), and we even have members in other European countries (multinational families tend to be mobile). We officially "founded ourselves" as a charitable, non profit self-help group at the beginning of 1996, although we had been active in the Rhein-Main area since our older kids were born in 1990. We're very grass roots, that is, our focus is on practical support (helping families to network, start up groups for their kids, find materials in their languages, etc.). The Polyglot (in parallel German and English editions) is our quarterly publication with articles, book reviews, and Internet tips of interest to multilingual families, as well as family contact and classified ads.
IDA HomePage (Interessengemeinschaft Deutschsprachiger im Ausland):
information for German-speaking expatriates
http://www.pcpublishing.com/ida/

ImAusland Mailing List: Discussion list for German-speaking people expatriates, run by the IDA (see above); subscribe by going to http://www.egroups.com/subscribe.cgi/ImAusland
and typing in your e-mail address and a password which you make up yourself.

IDAKinder Mailing List: Second discussion list run by IDA (see above), but dealing specifically with family-related issues (multilingualism, schools, etc.) of German expatriates with children; subscribe by going to http://www.egroups.com/subscribe.cgi/IDAKinder and typing in your e-mail
address and a password which you make up yourself.

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JAPAN

JALT - The Japanese Association for Lanuage Teaching

Check out their web-site with lots of links to bilingualism in Japan
http://www.kagawa-jc.ac.jp/~steve_mc/jaltbsig/index.html

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NETHERLANDS

"Dubbelop": Internet group (Steunpunt) for multilingual families with
Dutch/Flemish as one of their languages; an e-mail discussion list is
planned.
http://home.planet.nl/~stuur013

The French play group

A member of the mailinglist, Nathalie, started a playgroup for french talking children in The Netherlands. The group has got a website with among other things a list of recommended children books (+ a possibility to order them online).

http://home.planet.nl/~natmart

"Tweetalig" - A Mailinglist about bilingualism/multilingualism in Dutch

Petra started this list.  To subscribe to the list: send a message to tweetalig-subscribe@yahoogroups.com.
More information at http://groups.yahoo.com/group/tweetalig

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SCOTLAND
Tayside, Scotland (Dundee-Perth and surrounding region)
Claudia M. Viola
Mains of Murie Cottage
ERROL
United Kingdom
mailto:rviola@errol.u-net.com

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SWEDEN

There was an organization who's activities were recently discontinued,
FFF (Föreningen för Flerspråkige Familjer). For more information, please
contact

Sanne Wittfooth sanne.wittfooth@alfa.telenordia.se

Leena Huss Leena.Huss@multietn.uu.se

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USA
(Chapel Hill, NC; 10/98: Susan Dennis wanted to try to found a group in
Chapel Hill.)
c/o Susan Dennis
dennis@ils.unc.edu

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The Bilingual Family Newsletter is published quarterly in the UK in English
and is a good way to get current information. They also publish letters
from members which are answered by their knowledgeable board of editors.
Contact Marjukka Grover (mailto:marjukka@multi.demon.co.uk) and she'll be
happy to mail you a free sample copy.

1) Started by Steven Krashen and associates when the National Clearinghouse for Bilingual Education was forced to shut down its listserv.  For reasons we won't go into here :-(
Point your web browser to their homepage:
Once the homepage has loaded scroll down the left index to: "Multilingual Electronic Discussion List"  When this page loads it will explain how to subscribe to the list.

2) Started by teachers at the City University of New York. It now has several daughter lists. The main list is probably most appropriate for your query.
To subscribe, send a message here  saying: "SUB TESL-L first_name last_name"
Directions and a description of daughter lists will follow.