When the "set_param porject.enableVHDL2008 1" Tcl switch is turned on, a "Use VHDL 2008" switch will appear in the General Settings window.
If the Target Language is set to Verilog, this option will be grayed out.
To work around this issue, change the Target Language to VHDL.
Starting with Vivado 2016.1, the project.enableVHDL2008 switch is turned on by default. You will not see "Use VHDL 2008" switch in the General Settings window, but VHDL 2008 is supported by default.