> learn a language with very few speakers
Learn Sanskrit. You were writing in a descendant language just now. If you learnt the roots, you would understand the descendants better. And they are your tools, and knowing your tools is important.
Other more obscure languages? Well, as implied, languages are related... So studying the network clarifies the whole.
Sanskrit is not an ancestor of English. Sanskrit is a satem language English centum. Proto Indo Iranian diverged long ago.
Everyone should learn Sanskrit if only to read the great poets and dramatists in the original. The translations are often nice, but you do lose something in the process. Thanks to the incredible efforts of Pāṇini[1], the language was mostly frozen in time and is still just as understandable today as it was 2500 years ago.
I started learning it because it is India's civilizational language and I wanted to read the itihāsas in the original.
I have the Assimil Sanskrit course sitting on my shelf that I’ve been meaning to spend time with. Would love to be able to read old sutras one of these days.
Or learn Hittite. It's got the oldest corpus in the Indo-European language family. You've got your standard assortment of Indo-European cases. Grammatical gender is animate/inanimate and the animate gender has yet to split into masculine and feminine. The script is tons of fun, especially if you write it with a stylus on clay. And as a bonus, you get to learn a smattering of Sumerian and Akkadian vocabulary.