If you look at the documentation of the Unpack goal: 

< description>**Downloads and unzips** the SDK to your maven repository. Use this goal, if you don&apos;t wish to
specify a &lt;i&gt;gae.home&lt;/i&gt; or &lt;i&gt;-Dappegine.sdk.home property&lt;/i&gt;. The plugin will now search for
the SDK in that default location.< /description>

For a demonstration, I tried this:
* I erased my .m2/
* I ran mvn clean deploy

And here it is: http://gwt18nsample.appspot.com/

So, having the Unpack goal in the Deploy phase works for someone who has an empty .m2/.

I have to specify that for my i18n sample, I don't need a server (it's only a client-side demo). That's why I try to postpone downloading the SDK the most possible :P