You add the code to /etc/asterisk/extensions_custom.conf and set Context for your trunk to the name of your new context.
However, you may find a dynamic route easier. For example, see Using Dynamic Routes for Call Flow Control , or search for other examples.
In any case, you should allow easy manual language selection. For example:
“Bienvenue au camp de l’utopie. For English, please press 9. (Instructions continue in French.)”