If the board is spam, it will be muted, locked. Locals can continue their spamming before the board is locked.
Let me be specific, Nigeria local board consists of quality posters.
The board is in English and global moderators, current moderators who can use English can moderate it. All moderators in Bitcointalk can use English well and all of them can do this.
Some people prefer to use Pidgin, while some people prefer to use English, while some people prefer to use both.