So far we have trouble finding enough people to write (quality) English articles, so there is no point in starting complete wikies for different languages right now. Before we start writing all sorts of articles in languages different then English, we should start with explaining some of the basic DOs and DONTs in other languages.
Will will concentrate on translating stuff, that makes most sense! There is absolutely no point in translating device-pages! Don't even bother. There is also little point in translating complicated howtos, since the persons they are addressing, most likely understand enough English to get the work done. A translation of such a page, would bring only little benefit. It could be useful to translate pages like user.beginner, so people with insufficient English knowledge would at least get past the basics with the help of the documentation. The same is true for an article like obtain.firmware. A thorough translation should avoid misunderstandings.
Most tinkerers understand some basic English anyway, but sometimes the comprehension is (obviously) not that good, and misunderstandings occur. Then people start wasting time doing unnecessary and fruitless things and after a while they get frustrated. This is a pity. This is bad bad.
A translation shall help all the tinkerers without good enough English language skills out there avoid misunderstandings or lack of understanding and therefore frustration.
Another good reason to write an article in you mother tongue is, that you yourself are not proficient enough in English to write an article but still want to provide some documentation.
Do not translate things "to show the flag". Nobody gives a f*ck! Do it to provide comprehension of technical and software stuff. This project is all about cooperation and not about excluding each other.
The wiki is used by geeks and nerds and such to become. Predominantly they do have at least basic English language skills so there is really no use in translating all the wiki.
The WebUI is used by anybody predominantly non-geeks (= noobs?). So far, there are a couple of WebUIs available for OpenWrt! See overview over WebUIs. If you want to make it easier for end users, this is the toe-hold.