Add to this page: https://docs.magnolia-cms.com/product-docs/6.2/Developing/Templating/Dialog-definition/Field-definition/List-of-fields/Text-field.html

In the defaultValue description, draft from Adam:
-> i18n key could be used for defaultValue in order to have translations for default values in the form

defaultValue: some.i18n.key

-> in this case you need to configure

converterClass: info.magnolia.ui.field.converter.TranslatingConverter

...
For reference, config example at the top of the PR: https://git.magnolia-cms.com/projects/PLATFORM/repos/ui/pull-requests/2222/overview
(so contacts-v8.salutation.defaultValue=Herr is used for the defaultValue if the language is de in the example)