I had this same gripe in Origins playing as a Dalish Warden. There were never any dalish dialogue options, just Andrastian choices. One of the perks of my job, is that we do business with Hutterite colonies. The children only speak their variant of German up to age 6 or 7 i believe. At that time, then they are taught english. Primarily, if they aren't speaking directly to you, they will speak German amongst themselves. I imagine that upbringing would be damn close for Dalish Elves. Since both cultures are vigilant in preserving the past, speaking the language would be a top priority. The only reason why a Dalish Inquisitor can't (fully) understand the conversation between Solas and Abelas is because Bioware doesn't want to spoil the plot, reasons being,
1. A Dalish Inquisitor can understand the conversation between Solas and a Spirit of Wisdom, which is more likely, that a spirit of wisdom speaks ancient elvish or the dalish variant of ancient elvish? Most would say ancient elvish I presume, if a Dalish Inq can understand those two, he/she could understand the conversation between Solas and Abelas.
2. Playing devil's advocate, lets say that Solas and S.O.W. where speaking the dalish variant of ancient elvish, Even though the dalish variant is different than the language of ancient elvish, the roots of the language are based off of the ancient language. So a dalish inq probably would get an idea of what the two are saying to each other. The Dalish Inquisitor could ask for clarification if needed for a certain word or words in the converstation that didn't make sense. Even if the Dalish Inquisitor couldn't understand a word of the conversation, the most reasonable question would be to Solas, "How/where did you learn ancient elvish." If you don't like the answer he gives you,
3. You could ask Morrigan, she seems to have knowledge about the elvish language. Whether that is the dalish variant or ancient elvish, i don't know. But she could either tell you if she did or didn't understand the conversation between Solas and Abelas.