Clarify that BIDS specification is in American English

"Explicit is better than implicit." and question of the spelling does
come up from time to time, and ATM I observe BEPs prepared by European
Colleagues are using British English in their documents.

Making it explicit would help to minimize possible tensions and required
later work to prepare contributions into acceptable state.

src/common-principles.md

@@ -1,5 +1,9 @@
 # Common principles
 
+## Language
+
+BIDS specification is written in American English.
+
 ## Definitions
 
 The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",