truncate_bytes.rb

# truncate string by byte limit
#
# str - String
# limit - Maximum number of bytes
#
# Returns a String
def limit_string(str, limit)
  limited_string = ""

  str.each_char do |c|
    break if limited_string.bytesize + c.bytesize > limit
    limited_string += c
  end

  limited_string
end

limit_string("The biggest molecule", 20) # "The biggest molecule"
limit_string("Am größten Molekül", 20)   # "Am größten Molekü"

@InfraRuby, thanks!