Currently language handling is differing between legacy and Solr.
If legacy is changed to behave like Solr, we can remove use of the newly deprecated language parm for Field (and MapLocation) SortClauses which is causing queries to run slow on mysql.
In short this would be making language filtering behave more like legacy kernel*, only picking one of the translations in accordance to prioritized language mapping. This is already the case in Solr.
* see eZContentLanguage::sqlFilter: