How to reference value of field in translate?

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" ]
}