Talk about it. Prolifically. Say that you have no problem working on "that thing" but that it's going to take more time, because you're unfamiliar with that language / library / service or whatever it is that would slow you down.
Then adjust your estimates. If you are working on something completely new that you know nothing about, put in "Spike" tickets to prototype things. Put larger estimates for the tasks themselves after for the "unknown unknowns". When you discover an unknown unknown that can be extracted as a new ticket from what you are currently working on, create a ticket for it to make that fact visible and adjust your current ticket to say it got extracted so nobody still expects your ticket to do it.
Tickets are not evil, tickets are protection for developers - create them and comment on them, explaining what is going on and what needs to be done. Most companies aren't really "evil" in that way. They just don't know better. If you do happen to be in one of the 'actually evil' ones, go find another company to work for. E.g. if developer are not allowed by process or actually prevented by the ticketing software from just creating tickets and moving things around as necessary: run. Don't walk. Run from that company.