Trying to switch keyboard layouts in ThinLinc is very unreliable and will often simply not work:
a) The client layout is used, not the servers (see bug 4560). So if the client is full screen (with system keys enabled), then nothing useful generally happens. The layout might be switched in the session (depending on configuration), but that isn't really used so the user won't get what they want.
b) If the local layout is actually changed (or the session layout in a) ), then you get a client/session layout mismatch with can easily land you in problems because of bug 5269.
As noted on bug 5269, this worked a lot better on older distributions where there was more free room in the session keyboard layouts. If the client switched layout then Xvnc would generally be able to fit the second layout in to the free space. This is no longer the case with modern systems.
A workaround is to load one of those older layouts, or manually construct a layout that contains all the symbols clients might used.