If you're willing to transform your translate data a bit you could structure the file like:
Apple_12345: macbook
Apple_12346: iphone
Dell_12345: laptop
Then construct a new field to use for your translate:
translate {
add_field => { "lookup" => "%{brand}_%{part_number}" }
field => "lookup"
destination => "model"
dictionary_path => "translate_file.yml"
remove_field => [ "lookup" ]
}