I do not think the social media account of the Foundation should use any other language than English. This is because we need to target the largest audience (and left nobody on the side) and de-facto English the the common language we all use.
As we also accept other languages than English in the discuss forum, we may also use other languages on the social media account. Some social media has a translate button (like this forum) that can be used for others that do not understand the language.
I do not see any reason why the social media accounts should not represent the diversity we have in our community.
It is not the same to accept that externals are communicating with the project in their language and the project communicate only for a small portion of the community.
As long as the account also used English with the same content I don’t see an issue with the account using another language.
I think I’ve already seen the FSF, EFF or the PSF tweet in Spanish and then in English the same content.
Of course it should be limited to few posts, it don’t think it’s useful for the Tryton Tips for example.
(In Mastondon, the language can be specified for each toot and user can select the language they want to see).
I do not see the point because as @pokoli said in most platform followers can translate the post.
So having multiple posting of the same content will just create noise and annoy followers.
A quick check on the last months, I did not see any on the PSF, FSF nor EFF twitter account.
I can see one exception. For example when it is about a local event that will be in the local language.
Not every platform has this option.
The duplicate content is a minor annoyance from my point of view (the timeline of people have probably bigger annoyances such as advertisements, RT of political issues or whatever).
Yes they don’t do that often, but they do sometimes. With a google search I found some:
The first one is about a generic issue, the second is about a law in Mexico (so it’s understandable they tweet in Spanish). It’s not something the do every month but it happens.
Why are you asking? What is the problem?