Here’s a quick one-liner that can rename multiple files with PowerShell. The rename-item cmdlet can be used to search and replace text in the original filename.

dir "c:\Work" -s | rename-item -NewName {$_.name -replace "text-to-find","repacement-text"}

Here’s an example directory containing a few files that need renaming:

dir c:\work -s

    Directory: C:\work

Mode                LastWriteTime         Length Name
----                -------------         ------ ----
-a----       30/09/2019     21:11              0 01-Dunkelgrau-2019.bmp
-a----       30/09/2019     21:11              0 01-Kalender-2019.bmp
-a----       30/09/2019     21:11              0 02-Dunkelgrau-2019.bmp
-a----       30/09/2019     21:11              0 02-Kalender-2019.bmp
-a----       30/09/2019     21:11              0 03-Dunkelgrau-2019.bmp
-a----       30/09/2019     21:11              0 03-Kalender-2019.bmp
-a----       30/09/2019     21:11              0 04-Kalender-2019.bmp

Here’s a command that will rename some of the files, replacing “Kalender” with “Calendar”:

dir "c:\Work" -s | rename-item -NewName {$_.name -replace "Kalender","Calendar"}

We can check the results and see the file has been renamed

dir c:\work -s

    Directory: C:\work

Mode                LastWriteTime         Length Name
----                -------------         ------ ----
-a----       30/09/2019     21:11              0 01-Calendar-2019.bmp
-a----       30/09/2019     21:11              0 01-Dunkelgrau-2019.bmp
-a----       30/09/2019     21:11              0 02-Calendar-2019.bmp
-a----       30/09/2019     21:11              0 02-Dunkelgrau-2019.bmp
-a----       30/09/2019     21:11              0 03-Calendar-2019.bmp
-a----       30/09/2019     21:11              0 03-Dunkelgrau-2019.bmp
-a----       30/09/2019     21:11              0 04-Calendar-2019.bmp

The text to find is not case-sensitive so this works: 

dir "c:\Work" -s | rename-item -NewName {$_.name -replace "dunkelgrau","DarkGrey"}

dir c:\work -s

    Directory: C:\work

Mode                LastWriteTime         Length Name
----                -------------         ------ ----
-a----       30/09/2019     21:11              0 01-Calendar-2019.bmp
-a----       30/09/2019     21:11              0 01-DarkGrey-2019.bmp
-a----       30/09/2019     21:11              0 02-Calendar-2019.bmp
-a----       30/09/2019     21:11              0 02-DarkGrey-2019.bmp
-a----       30/09/2019     21:11              0 03-Calendar-2019.bmp
-a----       30/09/2019     21:11              0 03-DarkGrey-2019.bmp
-a----       30/09/2019     21:11              0 04-Calendar-2019.bmp

The replacement text is case-sensitive however, as can be seen in this example:

dir "c:\Work" -s | rename-item -NewName {$_.name -replace "DarkGrey","lightgrey"}

dir c:\work -s

    Directory: C:\work

Mode                LastWriteTime         Length Name
----                -------------         ------ ----
-a----       30/09/2019     21:11              0 01-Calendar-2019.bmp
-a----       30/09/2019     21:11              0 01-lightgrey-2019.bmp
-a----       30/09/2019     21:11              0 02-Calendar-2019.bmp
-a----       30/09/2019     21:11              0 02-lightgrey-2019.bmp
-a----       30/09/2019     21:11              0 03-Calendar-2019.bmp
-a----       30/09/2019     21:11              0 03-lightgrey-2019.bmp
-a----       30/09/2019     21:11              0 04-Calendar-2019.bmp

For more examples, see the PowerShell docs for rename-item.