When 5.3 was out, we introduced MultiLingual for 5.3, so by default all new fields that are created in a custom module, built with the module builder, are localizable. However the old fields are not converted to LStrings, but you have the option to convert them to such type. In order to achieve this, you can go to the backend page, which lists all fields for the content type of your custom module. Then if you edit each one of the fields, you will see an option:
"Make localizable" which when clicked will convert your old data type to Lstring. I have attached a screenshot of the location of the option.
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