The Languages Reference contains the available languages that are available to be set as "Preferred Language" in the Patient Tab. You cannot add or remove Language References.
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The "Show in Lists" checkbox allows you to add or remove the currently selected Language from the available options when selecting a patient's Preferred Language. If you need to enable a particular Language, first locate it in the sidebar, then click the "Show in Lists" checkbox. Don't forget to save by pressing Command-S.
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