Don’t quote me on this, I’m not a dev.
I imagine they would have to query for the last chapter in a given language for each manga on the list, which would put heavy strain on the database if it was done for every item on the list. Or add an extra field for the last chapter number in each...
Since they don’t want to add filtering manga by completed scanlation, they will be even less likely to add this.
Apparently it’s hard to filter only one language (English) from the chapter list for each series?