Cannot add translation to Content Type.

Steps:

1. Log in to admin panel.
2. Go to Admin / Content Types.
3. Select e.g. Content / Folder.
4. Switch to Translations tab.
5. Click Add translation.
6. Try to expand the Select a language dropdown.

Actual: It's not possible to expand the dropdown.
Expected: Translation can be added to CT.

Remarks: At some point it was reproducible at the level of Content Item but that stopped occurring.