ClassCastException w/ multiple <langusage> nodes

If an EAD looks like this:

```xml
<langusage>
    <language code="eng">English</language>
    <language code="cze">Czech</language>
</langusage>
```
...the result will be a `ClassCastException: java.util.ArrayList cannot be cast to java.lang.String`

We only support single-language EADs so this should be a validation error.