I purchased a Japanese product and then an English version was released later.

The work ID (RJ########) is the defining feature that sets one product apart from another.
* The work IDs of products sold on DLsite can be confirmed by looking at the URL of respective product details pages. The ID will be preceded by RJ/BJ/VJ, and then display a 6 or 8 digit number.

When a customer purchases something, it is the right to download / receive support for the data registered to such an ID that they are purchasing. This is important because, in many cases, the original Japanese version and the translated English version of works will be registered by the creator as entirely different products with different IDs.

• If the translated version of a product is registered under a different ID, then that means it is treated as an entirely separate product from the original Japanese version and the purchase of one language version does not give a purchaser access rights to the other.

For example: 
A customer purchases the original Japanese ver. of a product RJ12345678.
Then, the English (or otherwise ver.) of said product is released for sale at a later date.
* Does the above customer gain access to that new version free of charge?

The answer to this question is that... it depends on the product and there are no guarantees.

If the English data was added to that same product (RJ12345678) as an update, then yes, the customer would gain access. (They would just need to re-download the product after the data is updated.)
However, if the English data was registered by the creator as an entirely different product (RJ99999999), then no, the customer would generally not gain access to it. (There are sometimes exceptions to this, but they are few and far between.)

As customers have no way of knowing which works will be translated by who, in-house, out-house, 3rd party or otherwise... who will own the rights to those works, and what method the creator intends to / will actually end up registering translated product data... it is best not to make any assumptions about gaining access to future content.
 
Thus, DLsite recommends that customers only purchase products that they want to have "as is."

That is to say... no matter what a creator says or promises, or what rumors - or lack of - there may be about translations, updates, specials, etc... Please only base your judgement on the ~actual~ content of the product at the time of purchase.