Behaviour packs are sometimes "incorrectly" spelt "Behavior Packs" (which would be US English) in the UK English language file.

The spelling mistake can be found in the following translation strings:

• menu.behaviors
• progressScreen.dialog.title.behaviorAndResourcePack
• progressScreen.dialog.title.onlyBehavior
• progressScreen.dialog.message.onlyBehavior
• progressScreen.dialog.message.behaviorAndOptionalResourcePack
• progressScreen.dialog.message.behaviorAndResourcePack
• resourcePack.available.title.behaviorPacks
• resourcePack.available.none.behavior
• resourcePack.selected.title.behaviorPacks
• resourcePack.error.ingame.behaviorPacks
• resourcePack.description.bottom.behaviorPack

Source: the en_gb.lang file inside the template resource pack (which can also be found as an attachment)

P.S.: Technically it's not a spelling mistake, but US-American English (which seems a bit out of place since behaviour packs are spelt the British way in some translation strings)