language: unflattenLanguageSpec: no lonely '_' keys

This might be a crude implementation, but keeps language results
from including {_: 'some string...'} objects that don't have any
other keys, even when preventing so causes a mismatch with the
reference spec/layout. (Of course, the "mismatch" is only
presentationally and not semantically, and we already make other
presentational differences because we output JSON and not YAML
in this function.)

src/data/language.js

@@ -120,6 +120,11 @@ export function unflattenLanguageSpec(flat, reference) {
         return {[firstKey]: result};
       }
 
+      const resultKeys = Object.keys(result);
+      if (resultKeys.length === 1 && resultKeys[0] === '_') {
+        return {[firstKey]: result._};
+      }
+
       if (refKeys.length > 1) {
         return withEntries(result, entries =>
           entries.map(([key, value]) => [`${firstKey}.${key}`, value]));