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First, let me note as I should've earlier, your previous comments that DT Register links will not include the language tags was false. Even prior to us adding Falang, that has always worked. Also, DT Register in 2.5 has always responded accordingly to whatever language is set in Joomla. Based on the set language, the appropriate language files would be used.
Second, using Falang is NOT using something that is obsolete. That is your opinion, which is fine. It is an OPTION because the core multi-language feature does NOT work completely for everyone and many don't like it anyway as it involves so much duplication work with multiple copies of the same menu items... same articles, etc. Using Falang, you don't have to do all of that. Many people prefer Falang or Joomfish type setups. Not everyone does. Everyone is entitled to their own opinion, but that's what it is... opinion
We added Falang support because MANY MANY people have asked for it specifically.
The core multi-language system is intended for content articles, menu items, etc. It is not intended for full language control of other components. DT Reg will respond to your core language setups for the menu items and such... and will pull in appropriate language files for all general text... but what about the other database-driven elements such as field names, category names, location details, event descriptions, email messages, etc? To make all of those controlled by the core language system, it would complicate the management of DT Register elements beyond what is needed.
A Falang solution makes more sense in our opinion for DT Register. If you're just running a simple content-driven site, then use the core language functions on their own. If you go much beyond that, our recommendation would be to move beyond that. Thanks for your input!