fixed translator locale management
bug #830 fixed translator locale management (fabpot)

This PR was merged into the 1.1 branch.

Discussion
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fixed translator locale management

The locale used by the translator was set the first time the translator was instantiated. That's a problem for several reasons:

 * changing the locale via `$app['locale']` would not change the locale used by the translator;

 * when using fragments, the locale was not updated correctly;

 * also, when using the web profiler, the translator is created early on (before the handling of a request), and so the locale was always `en` (see silexphp/Silex-WebProfiler#23).

Commits
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902c2c8 fixed translator locale management