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.