This technique will work as long as you have some sort of server-side templating language available when serving/processing your JavaScript. When building a large application, you will want to concatenate and compress your JavaScript for optimal performance. We provide a tool to do this (JSBuilder2 http://www.extjs.com/products/jsbuilder/).

What I would suggest instead would be to place the default language into the prototype of the classes which you are developing. You can then dynamically generate a smaller file which includes your language specific text and overwrites the prototype. This is the same technique that we use to implement i8n in Ext and it works quite well. Check out the example in the SDK.
http://www.extjs.com/deploy/dev/examples/locale/dutch-form.html