language fallback: minor fixes

This PR intends to fix 3 small bugs:

* `lexicon` doesn't handle the case where `FALLBACK` is a csv string

* when setting `FALLBACK` to a language+coutry code, `FALLBACK` and `$fallback` get out of sync:
```php
$f3->set('FALLBACK','fr-FR');
//=> FALLBACK is actually set to 'fr-FR,fr'
//=> but internal $fallback is set to 'fr-FR'
```

* when setting `LANGUAGE` prior to `FALLBACK`, `LANGUAGE` is erased:
```php
$f3->set('LANGUAGE','it');
$f3->set('FALLBACK','fr-FR');
//=> LANGUAGE is set to 'fr-FR,fr'
```