I had a look at the Blender wiki.
Indeed, it is organized close to what I suggested above: same wiki, different namespaces
Look at http://wiki.blender.org/index.php/QuickStart you will see
This page in other languages … with links to QuickStart.fr, QuickStart.it, etc.
Without redirections, page names would be the same that the reference English page but with the page title in the Foreign language.
Doc pages in Foreign languages would have a link to reference English page.
As regards the screenshots, they can be the same since we are documenting the English Wikidot 1.0.
I don't think the 100Mb is an issue, at the moment.
As regards backup ? What backup ?
As regards separate wikis, I don't like the current tendency to multiply the Community wikidot websites: www, snippets, community, handbooks(?), admin… … because it is difficult to search them all, except with this, or to make linkages between them (no backlinks ?). Also, we would have one unique forum attached to the handbook with different language categories.
As regards the tags, they will be English tags related to English commands and modules.
The TagCloud module allows to filter by category, thus by language namespace.