This setting cannot be changed later, but you can always create and populate a new index with a different setting.

This setting will heavily influence the efficiency and matching of your index. There is a technique named "stemming" used to match words that have similar stems (i.e. provide and provided). The language setting controls how the stemming works.

This language is unique in that stemming gets turned off and the literal words are used. This should generally only be used when the items in your index are not words of any language, like usernames or email addresses, but can improve performance when this is the case.