I'm dreadful at languages, but I like learning about features other languages have.

Like the dual, or a distinctive 2nd person plural, or ways to say (that you have to use) how you know a a think (it was me/it was my friend/I heard it from some guy/I heard it in an authoratative source), or absolute (rather than relative) direction.

The language I learnt best was Latin (how useful!) but not conversationally, but a decade ago I could competently read Pliny (not any more, sadface, should read about Caecilius...).