Jeff and Joel talked about the importance of the big 5 languages and that these were worthy of their own sites because there are a large number of programmers who also speak these languages.
Well, first, that's not really what they said:
Jeff: There's only like five languages that are sort of big enough
Joel: In IT. There are 10 languages, probably, that cover a big chunk of the world's population. But in IT it's a smaller number. And the five that I was talking about are the five that I would want for programming questions, in those languages
These sites don't have anything to do with programming:
The English Language and Usage Stack Exchange is for linguists, etymologists, and (serious) English language enthusiasts
German Language and Usage - Stack Exchange is for students having questions about German, expert speakers of German wanting to discuss the finer points of the language and translation questions from any language to German.
I got the impression he was listing the proposals that are likely to succeed because A51 proposals tend to get a lot of traffic from SO, but they're not "worthy of their own sites"
Are there any suggestions on how these language sites could be better promoted in our programming community?
I honestly don't think programmers are likely to be interested in these sites just because they happen to be the languages used by a majority of programmers; they're totally unrelated topics. In the case of German, where the focus is on learning the language, it's exactly the opposite -- it'll be people who don't know the language posting the question
Am I allowed to create a unique ad for the Chinese site to be included as part of the Open Source Advertising Sidebar?
According to the instructions:
It must be an advertisement soliciting the participation and contribution of programmers writing actual source code.
So it sounds like no
Are there other ways to promote these sites in our community beside ad placements?
There's a few discussions on meta (and probably A51 itself) about how to promote sites. One example is What do you do to promote your Area51 proposal?