language property exported with str and dict values

I have a Plone 5.2 AT site that exports most of the content with

```
"language": ""
```

and some content items as

```
"language": {
"title": "Deutsch",
"token": "de"
},
```

This leads to import errors such as

```
2022-04-18 14:24:26,230 WARNING [collective.exportimport.import_content:352][waitress-2] cannot deserialize http://nohost/eteaching/projekt/rechte/hochschulmitglieder-als-urheber: BadRequest([{'message': 'Constraint not satisfied', 'field': 'language', 'error': 'ValidationError'}])
```

Start at collective.exportimport.import_content:352 and reproduce an import containing the two observed language representations. Trace how the language field is deserialized and validated, then verify that exported language values can be imported without the reported ValidationError.

Written by the indexing model from the issue text.