v3.10: Items op taal filteren (#26)

* (fix): remove duplicate line

* add "filterLanguageQuery()" query-helper

* add property AbstractRepository::language

* add property ItemsField::language

* enable filtering the connected items of a theme/subtheme by language

* add "language" of connected items in the API output

* bump version to 3.10; update change log

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Co-authored-by: Colin <colin@DESKTOP-IAN8NPS>