Changed default language codename can crash sync run

### Brief bug description

If a default language's codename from sync snapshot differs from the default lang codename in the target environment and, simultaneously, another language from the snapshot references the default language as a fallback, the `sync run` fails with `The specified fallback language is inactive or does not exist.`

This is most likely an ordering issue, where the languages referencing the default language are upserted before the patch operations renaming the default language take place. See screenshot for reference


### Repro steps

1. Have an empty target environment
2. Have a source environment where the default language codename is other than `default` and has other languages using the default language as fallback
3. Try syncing the models between the two environments
4. ???
5. Profit

### Expected behavior

...it works.

Start by reproducing the sync run with an empty target and a source whose default language has a different codename and dependent fallback languages. Trace the sync ordering around language upserts and the default-language rename, then verify that the run succeeds without the inactive-or-missing fallback error.

Written by the indexing model from the issue text.