If it’s always a specific future condition you want you could make a template sensor and use the new state_translated function, passing the new sensor to it.
Otherwise, if you’re just displaying forecasts on the UI, it should already be according to your locale.
How are you intending to use this? In a notification?
You can. Make a sensor with the raw condition value – or as many as you need. Then in the notification’s message template field pass the sensor to the function.
I’m not sure (I get the same behaviour as you). I think that maybe the translations are missing in the OWM integration’s translation files, although, I’d think that in this case the built-in HA translations for the weather integration would/could be used.
I searched the open issues for HA core on GitHub, but there’s nothing logged for state_translated. So, I’m either missing something or nobody has reported this as an issue.