Assume I have a simple interface for translating text (sample code in C#):

public interface ITranslationService
{
    string GetTranslation(string key, CultureInfo targetLanguage);
    // some other methods...
}

A first simple implementation of this interface already exists and simply goes to the database for every method call. Assuming a UI that is being translated at start up this results in one database call per control.

To improve this, I want to add the following behavior:

As soon as a request for one language comes in, fetch all translations from this language and cache them. All translation requests are served from the cache.

I thought about implementing this new behavior as a decorator, because all other methods of that interface implemented by the decorater would simple delegate to the decorated instance.
However, the implementation of GetTranslation wouldn't use GetTranslation of the decorated instance at all to get all translations of a certain language. It would fire its own query against the database.
This breaks the decorator pattern, because every functionality provided by the decorated instance is simply skipped. This becomes a real problem if there are other decorators involved.

My understanding is that a Decorator should be additive. In this case however, the decorator is replacing the behavior of the decorated instance.

I can't really think of a nice solution for this - how would you solve it? Everything is allowed, even a complete re-design of ITranslationService itself.