The problem you describe is not specific to the <Labels> data element or the translation mechanism, but rather to any model where a parent (or ancestor) reference to a data element is used.
The validation warning indicates that you're using (or rather reusing) the data element (in this case <Labels>) or a container of it (the translate model or a model containing it) in some context where there is no matching data element to reference.
Here's a simple example of the above scenario:
In this case validation will report an invalid reference in the translate model.
Note that identifying the exact reuse instance which causes this warning is currently somewhat cumbersome (see bug #580), as it may require the modeller to inspect (in a bottom-up fashion) multiple reuse cases in order to find it.
However once identified the resolution should be quiet simple, involving adding the missing data element to the relevant model. In the case of the <Labels> map it is even simpler, since translation is global, and once the <Labels> element in one views has been updates all other views will contain the update. In other words, the solution to your issue should be adding a <Labels> element to each and every view.
For best results, use the Firefox browser..