A few thoughts after thumbing through all 25 pages here:
1.) The juice is not worth the squeeze. A DM may craft a super complex language system with a huge number of languages, regional distinctions, etc... but in the end a first level ritual renders most of this complexity moot. As DMs we should make a consideration that every moment we spend crafting something PCs will bypass is a moment that we are not spending on elements they will enjoy more comprehensively. You as a DM may enjoy crafting the languages - and that is a great reason to do so regardless of whether you play D&D, but if you want to efficiently spend your time in a way that will most benefit your players, this is unlikely to get you that full glass of OJ you want.
2.) It is not just immortals that would stabilize the language - it is the presence of magic. We've seen movement towards global stabilization in the 125 years of modern technology we've had that have facilitated spoken word being transmitted across the globe ... in most campaign worlds this type of transmission of words has been possible through magic for tens of thousands of years.
3.) In my setting I have 'practical magic'. This is a recognition that it wouldn't just be adventurers that are having magic items made - it would be the rich and powerful merchants, nobles and royalty. Their needs would be different. They'd be far more like the magic we desire from our cell phones. Magic items that cast comprehend languages, light, message, sending ... and magic items that do what other technologies do or that people in power wish they could do : disguise self, charm person, command, dominate person, suggestion, unseen servant. A toy that casts phantasmal force on you so that you can experience an ... illusion ... of your choice. These magic items are not like technology that lasts 2 to 5 years - they last essentially forever. That means they'd pile up in cities or places where the long lived folk gather. Most wealthy homes have permanent unseen servants. The upper class show off their power and wealth by owning a plethora of magical gadgets. And in the end - with this practical magic floating around ... the importance of languages drops out considerably. YMMV - but this makes so much sense to me. And with all of this stuff available, an earring of comprehend languages is something a lot of PCs acquire early on.
4.) D&D is not the real world and language may not be the same. Orcs were created by Gruumsh. Did they have language from the moment of creation? If so, would that language be an inherent part of what an orc is? Do they really learn to speak orc as they age, or does the ability to speak it just manifest? YMMV if you have evolution explain the existence of all these species in your game rather than magic ... but in mine, this is the way. Orc is orc because Gruumsh made it orc.
