rovarspraket converter

rovarspraket.rb

def rovarspraket(string)
  vowels = ['a','e','i','o','u' 'y', 'Å', 'Ä', 'Ö']
  vowels.each{|vowel| vowel.downcase!}
  answer = ""
  string.each_char do |char|
    if char =~ /[[:alpha:]]/
      unless vowels.include?(char) || vowels.include?(char.downcase)
        answer += char + 'o' + char.downcase
      else
        answer += char
      end
    else answer += char
    end
  end
  answer
end

p rovarspraket("I'm speaking Robber's language!")