use I18n.locale_available? && syntax highlight

Hi @donaier & @effkay,

this changes

```ruby
if I18n.available_locales.map{|sym| sym.to_s }.include?(params[:locale])
```

just to the the functionality which rails already provides:

```ruby
I18n.locale_available? params[:locale]
```

and the language hints to the code blocks for syntax highlighting in github.

Thank you very much,
Alex