"Who are washing the brown dog?"
Translation:Ketkä pesevät ruskeaa koiraa?
It's still "Who is...". This is because 'who' is always singular. You only ever get 'Who are...' when 'who' is the acting object of the verb.
E.g. "Who are they?" - They is the acting subject here, so we use the plural form. This is shown in the answer "They are...".
"Who is washing the car?" - Who is the subject here, so we use the singular form because "who" is always singular.