DATAMONGO-1049 - Check for explicitly declared language field.

We now check for an explicitly declared language field for setting `langugae_override` within a text index. Therefore the attribute (even if named with the reserved keyword _language_) has to be explicitly marked with `@Language`. Prior to this change having: 

```java
@Language String lang;
String language;
```

would have caused trouble when trying to resolve index structures as one cannot set language override to more than one property.