A LanguageShield is some feature of a programming language put in to hide what you are doing, to give you protection, something to hide behind to give you room to change your mind, perhaps. As opposed to DesignShield, which serves the same purpose, but is not provided by the language but by program design.
Could it be that most Language Shields were Design Shields before they migrated into the language? See DesignShield for a discussion.
See also ShieldPattern by AlistairCockburn, where I first read the term "shield". -- FalkBruegmann
Some examples would be helpful. Perhaps ...
automatic register allocation
floating point representation
the mutable keyword (enables ConceptualConstness?)
Deep orthogonality and consistency in the language. For example: