Backend property editor does not support language tags

[gneissone]

Describe the bug
As described here: vivo-project/Vitro#296 (review)
The backend property editor does not support language tags. In fact, it actively removes them.

To Reproduce
Steps to reproduce the behavior:
Add data via the frontend. Language tags are added.
Edit data via the backend, language tags are stripped away.

Expected behavior
Language tags should remain when edited.
Alternatively, the form should alert the user of the form's behavior.

Screenshots
See vivo-project/Vitro#296 (review)

[litvinovg]

It also removes all labels but one if you try to edit label with backend property editor.

[gneissone]

Slight correction to my description; for labels it will add the en-US language tag no matter what the active locale is. It also deletes all other labels in other languages. For other properties, e.g. vivo:overview, it will remove the language tags.