Would be quite interesting to know the cause once this is solved
@Groatz
Did you check if the phrases were missing at compile time or are missing at runtime?
In many cases, phrases are compiled into the templates.
So if phrases were missing at compile time, the templates will contain the variable names instead.
I'd check one of the affected compiled templates to see if it does contain the phrase names instead of the actual text.
1) If it does, this would indicate that the phrases were missing at compile time.
2) If it does not, it would indicate that the affected phrases are dynamically loaded and are somehow missing/do not load properly at runtime
In case 2) I'd check the DB for one of those phrases if it is actually missing.
2a) If it is missing this would indicated that it somehow got deleted
2b) If it is not missing it would indicated that somehow loading of the phrase does fail
In case 2a) I'd check MySQL binary logs to see when the phrase did get deleted or renamed and try to correlate events.
In case 2b) I'd check the code path for loading the phrase to see what is going wrong.