Well, the point is make sure you Internationalise everything, not just the text you have in the templates.
For example, when using ajax calls you may be rendering text directly from actions, or you may be throwing exceptions which have meaningful error messages which are displayed by the template. You need to remember to “i18nalise” this text too…
From an action, you can access the i18n function via the context singleton, as documented by the Symfony docs (http://www.symfony-project.org/book/1_0/13-I18n-and-L10n)
Remember to do it now, and you won’t kick yourself later!