minetest CSM which records sent and received messages in the minetest log file.
tested with minetest 0.4.17 and 5.0 through 5.3-dev-ee831ed6e.
only basic CSM need be enabled in 5.0, which is the default.
make sure the mod is installed at ~/.minetest/clientmods/chatlog
make sure ~/.minetest/clientmods/mods.conf exists and contains:
load_mod_chatlog = true
logs will show up in ~/.minetest/debug.txt
if you use another CSM that handles the printing of chat messages e.g. friendly_chat or chat_highlights, you may have to put an entry like
chatlog?
into that other mod's depends.txt file.