[timepoint_list] Translate 'Unknown' entry in Language column

[HachemJ]

Brief summary of changes

This PR updates the 'Unknown' value in the Language column to be translatable.

Testing instructions (if applicable)

1. Go to 'Candidate' -> 'Access Profile'
2. Click on any candidate’s PSCID.
3. If the candidate has no visits/time points, go back to Step 2.
4. Switch the language to French.
5. Verify that the 'Unknown' value in the Language column is translated to 'inconnu'.

Link(s) to related issue(s)

• Resolves [timepoint] Multi-lingual - some of the texts are not translated #10627 (Reference the issue this fixes, if any.)