When I say "Translate English to Spanish" Siri opens the app Google Translate.
How does Siri know to launch this app and is it possible to register your app for certain keywords with Siri?
Usually you can open an app telling Siri something like:
"Open [APP NAME]"
In your case I guess Siri is not interpreting the sentence. When you say Translate English to Spanish Siri does try to understand what action should take and since no action with an hight Matching Rate is found then Siri tries to open the App having a name equals to the first word of your sentence.
In Italian language the app is named Traduttore. When I say Traduci dall'italiano all'inglese (Translate English to Spanish) Siri does NOT open the translator because Traduttore does not match Traduci.
If I say: Musica dall'italiano all'inglese (Music italian to english) which is not a real command, Siri does open the Music app.
Right now the interaction between Siri and third parties apps is pretty limited, however maybe in the future Apple will provide the tools for a deeper interaction.
Maybe something like what is happening with HomeKit where Apple created an interface to make Siri and Home Devices to talk each other.