I know how it feels when waiting for somebodies support to handle an API bug while your application is going to not met deadlines defined. The issue you described really sound like a bug, so for “clean” solution you will have to wait until Google Sites team guys will resolve this bug (I already upvoted 🙂 ) and you will be able to simply use the Search API.
However, in a meanwhile, I think you should try some workarounds. I may suggest you a different solution which will not met your needs for 100% but may be useful. For example, configure your site to expose the aggregation data feed to feed processor with rich search API – it may be an RSS feed with all articles from your Google Site burned by Feedly which have a nice multi languages search API support (Search the content of a stream) along with strong authentication to protect your data privacy.
As an architect I know that this is not a proper solution for the issue, but once it helped me to build a fully searchable application aggregating data from 100+ different data sources using russian and ukrainian locales.
Have a good luck in your application development and let me know if this solution helped you! 🙂