Is it feasible to circumnavigate the Earth in a sailplane?
No. Both night and oceans would be major obstacles.
Night time can be survived by parking the glider in ridge lift but it requires a steady wind during the night. This is possible and has been demonstrated decades ago, but is only possible with the right weather conditions. To have those lined up between thermal-powered daytime flying is rather unlikely.
Oceans are too homogenous to create thermals. You will notice that when crossing a large body of water in a glider: Over water the air is calm while over land it is easy to find thermals. Why else would albatrosses resort to dynamic soaring if the much less stressful thermalling would be possible? Dynamic soaring works much less well for human-sized gliders because of the limited height of the wind shear layer above the ocean's surface. Besides, it is beyond normal human capability to fly intermittently at 3g over many hours without sleep.
If you start high enough, a large lake can be crossed since the lack of thermals means you also will not hit downdrafts and your glide ratio will be easy to predict. But no thermal will carry you high enough to hop between Pacific islands or from Scotland to Iceland, which is a nearly 1000 km overwater trip.
For a true circumnavigation you would need to touch the southern hemisphere and there are too many long stretches of water to plan any feasible course. Only flying across Siberia and on to Alaska, Canada and Greenland does not make it a circumnavigation. Also, I very much doubt you will find thermals over large bodies of ice and snow, too.