Imagine a person lives in Spain, speaks only Spanish and Catalan, but is a great content creator and local board contributes (suppose guy is good at coding). Why he should be not welcomed in your campaign?
You have good point, but looking from advertisers perspective, they're focusing on English speaking boards to get most visibility. Hiring someone who only posting in local boards will be less cost-effective.Some campaigns pay for local posts, others don't, and my opinion is that it makes sense to pay for part of the posts in local boards as well, because there are some communities that are quite active and can promote the project at the local level. Of course, the key is to find the right ratio of posts in the general/local forum, and the more active the user is in several boards, the more visible the signature will be and the better the results for the campaign.
All in all, I think that managers in sig companies do a good job, at least when it comes to campaigns that pay exclusively in BTC, I can't say anything about bounty campaigns because I don't know anything about it.