Cardiff at 11pm can be interesting, exciting or fear inducing... Rather depends which bit and where you are going. I would say it's probably safe to walk 1 km however that really depends on you.

Walking around the centre is the only way to go... There may be bike hire places around but you shouldn't need to do that.

The weather - if it isn't raining it will be now in a minute. Actually yeh follow the weather forecast.

Local cuisine? Lol. Nope. Can't recommend anywhere. Depends what you call local.

Plenty of history around. The national museum is excellent.

Yeh nearly everyone speaks English in Cardiff. Even 1st language Welsh people (rare in Cardiff) will be fluent English speakers (except in north Wales where they might not be, or at least pretend not to be)

General tips - it can rain quite hard for quite long periods of time so be prepared to get wet, however it can also be dry and warm if not hot. Don't drink too much Brain's Dark, you will get a headache.