At least with that lookup table below, you can detect isolated continuation bytes (192 and 193) and invalid codes (above 244).

__gshared static immutable ubyte[] charWidthTab = [
            1, 1, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
            2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
            3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3,
            4, 4, 4, 4, 4, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1
];

length 5 and 6 need not to be tested specifically for your goto.

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