Steps to reproduce:

1. Move to "Maintenance" page in Configuration
2. Click "Create maintenance period"
3. Move to "Periods" tab
4. Click "New"
5. Choose "Monthly" from "Period type"
6. Click "Day of week" in Date
7. Choose "last" from "Day of week"

Result:

A word "last" translated to "最新値" that means "latest value" when proceed following the above steps in Japanese Language.

Expected:
In this case, I think "fifth" is correct word more than "last". "fifth" can translate to "第5" that means "fifth of week".