I think in an ideal world, profiles would be written in the language in which the person spoke, or with the location they are tied to.
However, one thing I think we should never do is try to create a biography based on translations. For example, writing a bio and using an online translator can have disastrous consequences.
While I have experience with writing and reading in Czech and other Slavic languages, I am nowhere near proficient in them so I write all biographies strictly in English. If I run across a profile in another language, I just fo a right-click -> Translate to English in Chrome and there I am, reading the bio in English.
It may not be perfect, but it works...
So, to answer the question:
In this process, I have run across other profiles, some even created by Spain Project members, and they are all in English. Is that the standard for Spain? It would sure save me a lot of time and work if that were the case, and the bios would look a lot more accurate.
Not speaking on behalf of any project here - but I don't think there should be any forced standards by projects to set biography languages. It really needs to depend on the writer (us), the considerations for the profile (who are we writing about), etc.
If the Spain Project forced Spanish, Catalan, Basque, etc. then it would be nearly impossible for 99% of the community to engage with the profiles and contribute to them.
For instance, my family came from Moravia and only spoke Czech. I still write their profiles in English - because it would be nearly impossible (and overly time consuming) to try and write it in Czech with my limited abilities.