Yes, spellIDs are showing in the combat log now and using spellIDs instead of spell names allows for de-localized implementations of anything that involves spells.
Basically one should now always use spellID and only get the name via the spellInfo API when wanting to display the name. Matching really should be done on an ID level. It will then automatically work for any locale.
What are the spell-id's actually? Some Blizzard internal identifier? I noticed they seem to match the spell-id's that for example wowhead.com uses.
Yes, the client data files contains a database (called spells.dbc IIRC) where all available spells are indexed using an identifier. This identifier is used by wowhead and wowdb, and evidently by the game engine itself.