I started wondering about this when I was on the French language StackExchange site, where I realized that the interface isn't available in French yet. This got me thinking that the travel StackExchange site is even more so international, since, well, it's about travel. Therefore, shouldn't you be able to post your questions/answers in whatever language you choose? It never mentions explicitly anywhere that you can't, and also I think some travel terminology can be difficult to translate sometimes. Furthermore, some non-native-English speakers might not be able to express their question as clearly as they would like to in English.
I'll agree with pnuts, with a caveat.
An entire question posted in a different language will get closed, because the community typically decides on these things and that has been the precedent until now. Do keep in mind there's an expectation on SE sites that a minimum of effort has been done before asking, so expecting someone to translate an entire question for you won't be seen kindly.
Also keep in mind, this site serves more than just you and I and the handful of other polyglots on the site, it's very highly ranked on Google. Not only that but by posting in English you maximize your chances of getting an answer since the set of English-speaking users is likely a superset of those who speak other languages.
However, I think that collectively here, we speak a lot of languages and there's a good chance someone could help translate a certain expression/word you're having trouble with. Especially since we don't get the volume of questions SO does (or even French, I think), I think a minor accommodation is not unreasonable.
I do remember that meta from French.SE and my opinion of that proposition hasn't changed since then: it's ineffective.
The language of the network is English except for a few Stack Overflow ‘duplicates’ that are in foreign languages — Portugese, Spanish, Russian and Japanese at the time of writing this to the best of my knowledge.
However, there are also a number of language sites each with a specific language, e.g. German, Japanese, Chinese, French, Italian … On these language sites, it is permitted to ask a question in the respective language. I saw an SE employee once state over on Area 51 that internally, this is justified with full immersion which in turn is known as one of the best methods to acquire languages. Since from an English point of view the language sites are language learning sites (at least to a specific extent) this makes sense.
In turn, this means that the sole language that should be spoken on Travel.SE is English as we are not dealing with a language. Questions only written in a foreign language will likely be closed as unclear what you’re asking in a glimps mainly because most users will not understand your language. You are expected to show prior research which would also include thinking of how to write in English.
Note that your English does not have to be perfect. Quite on the contrary, we get a number of posts with rather poor English, but as long as the gist can be understood this can be edited into better English by members of the community. Likewise, if you were talking about something that happens to be important in a foreign country (Halbtax-Abo?) you can include the foreign term as a technical term and do your best to explain it. If an established English word exists, someone will edit accordingly.