Hallo,
Preamble 1: This email probably falls under this FAQ question: Q: How will Wikidata change the way articles are edited? A: That’s part of what we have to figure out during the development, together with the community.
Preamble 2: It's possible that there's an answer to this issue already, but I couldn't find it.
A popular example of using Wikidata is that it makes maintaining articles about cities easier: When a mayor of a city changes, it must only be updated once.
The problem is that the mayor's name can be written differently in other languages. I didn't actually try running it myself, but as far as I understand, Wikidata supports translating names. But what happens when the mayor changes? It is likely that the name will be updated in the language spoken in that city. At that point articles in Wikipedia in other languages will probably show the name in the language of the city, which may be unreadable.
Let's take Haifa for an example. Its previous mayor was: he: עמרם מצנע en: Amram Mitzna ru: Амрам Мицна hr: Amram Micna etc.
Now it changes to: he: יונה יהב
And then suddenly all the articles about Haifa in all the languages will show the mayor's name as "יונה יהב", which most people won't be able to read. Maybe the Wikidata community will develop some kind of a policy that will discourage adding names in local scripts without any translation to a more common script. Maybe at some point software should even show a warning if somebody tries to do it.
The scenario can be even simpler: Somebody will vandalize Wikidata and change the mayor's name to some nonsense.
The most practical way to solve this is to show that some piece of data that affects a Wikipedia article in the watchlist, as if it is a change in the article itself. Is it possible? If not, is it planned?
It's a problem with Commons, too: An image that is used in an article can change in Commons and it won't appear in the watchlist. But I expect that it will happen a lot more often with Wikidata items and that the changes would be a lot more subtle and hard to notice: It's easy to notice that an image changed, but it's harder to notice a change in a number or a name of a mayor.
Another question is: What is the fallback mechanism if a name was not translated? The usual MediaWiki fallback rules can be reused, but there's a twist, because in Wikidata the usual fallback language may be unavailable. So in this case it will probably be:
my language -> my fallback language -> English -> the language in which it is written
-- Amir Elisha Aharoni · אָמִיר אֱלִישָׁע אַהֲרוֹנִי http://aharoni.wordpress.com “We're living in pieces, I want to live in peace.” – T. Moore