Hello,we were lucky enough to been here pre Brexit and our application is the pipeline,but we have read that people who are still to come have to start their application from the UK,and speaking to them, they have spoken to Various Spainish Embassy,s in the UK and have had various answers , from never heard of that before, to yes you need to answer all the form,s in Spainish,has anybody on this site been through a post Brexit application ?. We only ask this because some our very dear friends are now faced with this situation,and need know what is the best way to go.Any answers however painful will help them.TIA.

You need to look here.
And if other 3rd countries are the same then yes all the paperwork has to be in Spanish (not sure about the form itself though).

Visas (FAQ)

If you go to the bottom of the page it shows the visa forms and what you need. All paperwork needs to be translated and signed as accurate.

As far as Im aware the consulates are not open so no body has successfully applied yet (or maybe they have but are not on the forum's)
Just checked and the consulate in London is closed until further notice.

Your friends are aware that the application must be submitted in their home country and approved before they come here?

Hope this helps and can I ask that you keep us updated as to their progress, the information might help someone else.