Hi,
follow these steps:
- Change name from "Date of Birth" to "Date_of_Birth" because Joomla language constants does not allow white spaces
- Go to Extensions->Languages->Overrides
- Click to "New" button
- Fill the form, Language Constant (it will be "Date_of_Birth"
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and Text (your translation)
- Repeat this step for all languages, you can select language from filter bar (only "site" languages), see screenshot