This might be a duplicate since it's hard to search for strings like this on jira.

Anyway, when you successfully load a structure, the following messages are printed:

commands.load.success
commands.load.queued

These are the wrong translation strings, the correct strings are

commands.structure.load.success
commands.structure.load.queued

which are translated to

Loaded a structure of name %s
A placement request has been queued and it will be executed when the specified area is fully loaded.

respectively.

How to reproduce

1. Save a structure
2. Use /structure load to load it
→ commands.load.success

Notes

Hilariously, there is also a key for commands.structure.unloadedChunks which states "The feature to load a structure in an unloaded area hasn't been implemented yet," which is not true