It looks like the language files were not installed correctly. Please try re-installing the component: download the package again from https://www.AkeebaBackup.com/latest and install it using Extensions, Install/Uninstall. Please do not uninstall your existing copy of Admin Tools.
I would also like to note that this issue is purely cosmetic. What happens is this: some visitors are attacking your site repeatedly, triggering the Web Application Firewall (WAF) feature of Admin Tools. As a result, Admin Tools "gets angry" and bans their IP addresses for a preset amount of time. When using the default settings, if a visitor tries to attack your site three times within five minutes he's blocked for a whole day - if I recall correctly. You get this email when Admin Tools does this to protect your site, so essentially it tells you that Admin Tools does work and, in fact, works hard to protect your site.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!