Hi,
1 Given the fairly small amount of url's the easiest way to check is to do a site:yourdomain.com & see which url is missing (normally you should see 23 results here).
2. Google works in mysterious ways - quite possible that the page in the other language is dropped next week, or just remains in the index
3. Probably Google decided that this particular page wasn't interesting enough or considered it a duplicate of another page of your site.
4. Improve the content on the pages is probably the best way to get pages in the index. Must say that most sites don't get to 100% index ratio of the url's in their sitemaps. It's quite possible that the page that is dropped is something like a 'contact us' or "disclaimer' page which has no value for search.
In general, the number of url's of the sitemap that are indexed is just giving an indication how good your site is indexed. Being in the index is just the first step, it's the ranking & especially the search traffic that count. For one url dropped I would not put too much effort in figuring out which one it is.
Hope this helps,
Dirk