I guess the proper way of fixing this should be moving spider errors from spd_err.h to errmsg-utf8.txt, which will also make it automatically localised.

no, the proper way would be to extend plugin API to allow plugins to have their own translated error messages.
Proper plugins should have nothing outside of the plugin source directory, and in particular should not leak into server's errmsg-utf8.txt

serg I assume this rule applies to dynamic plugins only, i.e. those not in mysql_mandatory_plugins and mysql_optional_plugins, right? I mean there are plenty of things in the server code helping out builtin plugins like InnoDB, perfschema, aria etc.

it's a general rule with exception. there are few plugins who are more integrated into the server, unfortunately.
unix_socket plugin is compiled statically but it doesn't leak into the server. And rocksdb is compiled dynamically, but it does.

Spider could be, technically, compiled statically too, if you remove MODULE_ONLY from its CMakeLists.txt

> Spider could be, technically, compiled statically too, if you remove MODULE_ONLY from its CMakeLists.txt

or do PLUGIN_SPIDER:STRING=STATIC in CMakeCache.txt

Another example here.