Acting as the go-between for a language version is mostly a chore. If you are starting an expedition, please feel free to put your name in as go-between; it is not immodest or rude. Don't bother with voting or an election unless more than one person wants the job.
Don't sit back and wait for other users to sign up for the expedition; go out and recruit them. The language's Wikipedia version, Slashdot-like sites, blogs in the language, or Web forums are good places to recruit expedition members.
If the new language uses formal speech, decide whether to address the authors/readers in formal or informal speech.
If the new language has regional or country-by-country variations (like English, French, and German do), work out how you're going to deal with them.
Make a translation list of all article names in the Wikitravel: namespace. If every translator translates the linked article names on his own idea, it will become very hard to fix broken links after all. (This experience has been made at the start of de:) en:Wikitravel:Namespace index is a good place to start.
Make a translation list for all headers used in the article templates. Think twice for good translations. Very hard to change later.
Translate. If you are impatient enough to begin your translations on en: before launching the new version, all your links will be broken since you cannot write the translations into the en:Wikitravel: namespace. Thus, there is no control for misspelled links. Do not try to link to already translated articles in some user's sandbox, because after transferring them to the newly launched wiki, you will have a big mess to adapt them.
Note: the LanguageXX.php files used by old versions of MediaWiki are not used anymore, so please do not waste your time translating them. The basic interface for all major languages is built-in, but they can be customized online after launch by accessing MediaWiki:Allmessages (administrators only).