conversation: prevent language code mismatches pulling in builtin intents

[charludo]

• un-normalized language, e.g. en-AU, passed to HA to return list lof intents -> works, since HA normalizes internally
• un-normalized language used to fetch the list of builtin intents -> returns empty set, because the language was not normalized
• an intent is "custom" when it is not in the builtins -> all intents misclassified as custom
• startup check and pool building both explode in size and CPU time

Fixed by normalizing the language.

Fixes: #21