Since 2016-07-12 there is a surge in the number of Content Translation saving errors with the "readonly" status.
"Saving" here doesn't mean "saving a MediaWiki page", but "saving a translation in progress in the wikishared database". (If I'm not mistaken, this is done by using the cxsave API action.)
These errors are logged in EventLogging. An example of returned JSON trace is: {"servedby":"mw1207","error":{"code":"readonly","info":"The wiki is currently in read-only mode","readonlyreason":"The database has been automatically locked while the slave database servers. (Unfortunately it's truncated because of a limitation on string length in EventLogging.)
From 2015-11-01 until 2016-07-12 there were usually 0 of these errors every day, and sometimes a small number per, up to 11. Since that date it's between 30 and 90.
@Reedy and @Nikerabbit found that the error probably comes from LoadBalancer.php, but further investigation is needed: Why did this start happening? This is probably not a normal condition, and this affect the general stability of Content Translation and possibly other Wikimedia sites' features.