Putting this function in acs-lang looks like a good idea. Below is the current version, which adds a fall-back to en_US if a translation is missing.
CREATE OR REPLACE FUNCTION localize(TEXT,TEXT) RETURNS TEXT AS '
DECLARE lang_tag TEXT;
s TEXT;
package TEXT;
key TEXT;
r RECORD;
BEGIN
s := $1;
lang_tag := substring (s FROM ''#[-a-zA-Z0-9_:\.]+\#'');
WHILE lang_tag IS NOT NULL LOOP
package := substring(lang_tag FROM ''#(.*)\\\\.'');
key := substring(lang_tag FROM ''\\\\.(.*)#'');
SELECT INTO r *
FROM lang_messages
WHERE message_key=key AND package_key=package AND locale=$2;
IF r.message = '''' OR r.message IS NULL THEN
SELECT INTO r *
FROM lang_messages
WHERE message_key=key AND package_key=package AND locale=''en_US'';
END IF;
s := replace(s,lang_tag,r.message);
lang_tag := substring (s FROM ''#[-a-zA-Z0-9_:\.]+\#'');
END LOOP;
RETURN s;
END
' LANGUAGE 'plpgsql';
