I need to create a custom multi field widget in a metadata schema which allows user to choose a language from a drop down list and add a corresponding title for the language in the text box. With every language selected a text box should appear.
Refer - Creating custom widget in metadata schema
This would provide an idea on how to code for your custom requirements - Experiencing Adobe Experience Manager - Day CQ: AEM 62 - Add Multifield to Assets Metadata Schema Ed...
Ignore the fact that its 6.2, you can convert the specifics to 6.3/6.4 per your use case.