A "Long-Term Treasury" fund typically holds US government debt with a long expiration date, for example, 20 to 30 years in the future. It is frequently cited as a useful portfolio component for example in a "60 / 40 Portfolio" (where the 40% is treasuries).
Two funds that offer this US-specific portfolio are VGLT and TLT. However, it seems difficult to easily buy long-term government debt from non-US countries.
How can I gain exposure to long-term government bonds from many countries around the world?