You can open the custom dictation interface to specific languages via actions. There is an example action for Italian in the directory which can be easily modified for other languages. You can add these actions to a Home Screen widget to easily launch the app directly to dictation in the language of your choice.
Also, the locale selection in the dictation interface remembers your recently used locales, so they are at the top of the selection list…so should not be too painful to re-select your commonly used options.
As far as mixing languages, that would be up to Apple to implement in their dictation engine.