Sorts are always by priority (the order)
You sorted by series so Language is lower priority
If you want by Language ,That needs to be the last (most current sort in the Library view) . Calibre biases sort by the previous sort.
Sort by Title, then by Author and you will see that authors books in Title order.
Also, you can't have conflicting sorts (that work
)
You can't sort by Title and series_index (unless it is the Alphabet Murders
)