Open Extensions manager and just search 'Highlights Multibyte Symbols'. or clone the extension into the user subdirectory:
View menu and check
Highlights Mutibyte Symbols, or key shortcut
Ctrl-Alt-W is also available.
Ver.1.x.x highlights multibyte alphabets(Ａ-Ｚａ-ｚ), numbers(０-９),and below symbols.
If you need to know about original features and technical details, visit DennisKehrig's page.
Conflict with Show Whitespace when install.
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