You want to remove English as logon language for C4C.
Reproducing the Issue
1) Go to Business Configuration
2) Go to Edit Project Scope
3) Go to Step 4 - Questions
4) Select Built-In Services and Support -> Business Enviroment -> Adresses and Languages
5) Notice the question "You can use English as solution language" greyed out and checked
English is the default language, being used by SAP internal support and development systems. It cannot be removed.
This is standard system behavior.
KBA , AP-RC-BCT , Business Config. Tools (SAP Business ByDesign , How To