From what I have read I assume the following.
If Bing doesn't know your system language it assumes you're searching in your native language.
All the spotlight information is just based on a search using the file name or title and the Bing system.
If that title is in a different language it does as Bing would do and serves results in that language.
Then when you click on any information on the Spotlight screen the bing results are once again served in the language the search term was in.
So this file has a name or title in Thai, therefore all search results will be in Thai.
Just a flaw of using the Bing backbone to serve this information.