How does caching work?

If I have apc and memcached installed what is used by transposh? Is there a way to check if memcached is being used? I find mysql queries to be really slow how can I improve them? I guess I can’t?
How does it interact with w3tc? Pages are cached normally from w3tc after being translated?

https://ww.wp.xz.cn/plugins/transposh-translation-filter-for-wordpress/