Mosfet as a switch (on/off) and ground isolation
You can't claim &quot;full galvanic isolation&quot;.

Because there's no such thing. Legitimate isolating products claim things like &quot;galvanic isolation to 6000 V&quot; or &quot;galvanic isolation to 10,000 V&quot; and cite a relevant standard such as UL1577 or IEC 60747-5-5 that sets out how that isolation is specified and tested.
Even these products might fail to provide isolation if too high a voltage is applied between isolated terminals.

Because a MOSFET alone is not going to provide this kind of isolation. A high enough voltage (more on the scale of 100 V than 5000 V) will cause breakdown of the device and make hazardous voltages accessible on the other terminals of FET.
Maximum \$V_{ds}\$ and \$V_{gs}\$ values will be specified in your MOSFET's datasheet, and unless you're looking at some exotic device designed for use on the power grid and costing $100's, the limit will be much lower than the isolation values appropriate for galvanic isolation.



Is there any leakage current or high frequency noise passing through a switched off mosfet?

This is irrelevant to the question of galvanic isolation.
But yes, high frequency signals can be coupled between the gate and drain or source, since they're only separated by the gate capacitance.
And leakage current will flow from drain to source (or vice versa). This will be specified in your MOSFET's data sheet.