What is this translucent heat-resistant insulation?
That looks very much like PTFE tubing. The thin-walled tubing tends to be translucent, but the degree of translucency also varies by manufacturer and even batch.

PTFE can operate up to 260°C continuously. Transformer tails like this are typically tinned onto the bobbin pins by solder-pot dipping, we use 255°C for our lead-free solder pots and the sleeving does not directly touch molten solder which leaves enough margin.
As well as soldering you have to also remember that the maximum internal temperature of these type of high-frequency transformers can exceed 100°C which will limit your sleeving material choice too.