Language files of extensions, which start with:
are included into phpBB automatically to provide language keys for navigation, logs etc. within the ACP.
If you use "$phpbb_log->add" with 'mode=mod' the log entry with its details and action will be visible in "ACP > Maintenance > Moderator log" and via: "MCP > Moderator logs".
This leads to the fact, that you need the language key in the ACP and the MCP.
At moment the info_mcp_ is not enough, you need the same entry in info_acp_ as well.
So i think, the info_mcp_***.php needs be included in the ACP as well or similar.