ChineseFallbackFont.m

#import <Foundation/Foundation.h>
#import <AppKit/AppKit.h>

int main(int argc, const char * argv[])
{
	@autoreleasepool {
		NSLog(@"languages: %@", [NSLocale.preferredLanguages componentsJoinedByString: @", "]);
		
		NSMutableAttributedString *string = [[NSMutableAttributedString alloc] initWithString: @"暗a"];

		[string addAttribute:NSFontAttributeName value:[NSFont systemFontOfSize: 12] range:NSMakeRange(0, string.length)];
		NSFont *usedFont = [string attribute:NSFontAttributeName atIndex:0 effectiveRange:NULL];
		NSLog(@"orginal: %@", usedFont.fontName);
		NSLog(@"latin: %d", [usedFont.coveredCharacterSet characterIsMember: 'a']);

		[string fixAttributesInRange: NSMakeRange(0, string.length)];
		usedFont = [string attribute:NSFontAttributeName atIndex:0 effectiveRange:NULL];
		NSLog(@"fixed: %@", usedFont.fontName);
		NSLog(@"latin: %d", [usedFont.coveredCharacterSet characterIsMember: 'a']);
	}
	return 0;
}

Console output 1

2020-06-20 01:26:21.673229+0200 fonttest[40301:2583659] languages: en-US, zh-Hans-DE, ja-DE, de-DE, pt-DE, ru-DE, tr-DE, th-DE
2020-06-20 01:26:21.850740+0200 fonttest[40301:2583659] orginal: .AppleSystemUIFont
2020-06-20 01:26:21.851591+0200 fonttest[40301:2583659] latin: 1
2020-06-20 01:26:21.897532+0200 fonttest[40301:2583659] fixed: .PingFangSC-Regular
2020-06-20 01:26:21.897893+0200 fonttest[40301:2583659] latin: 0

Console output 2

2020-06-20 01:26:52.231687+0200 fonttest[40345:2585341] languages: en-US, ja-DE, zh-Hans-DE, de-DE, pt-DE, ru-DE, tr-DE, th-DE
2020-06-20 01:26:52.258971+0200 fonttest[40345:2585341] orginal: .AppleSystemUIFont
2020-06-20 01:26:52.259851+0200 fonttest[40345:2585341] latin: 1
2020-06-20 01:26:52.274636+0200 fonttest[40345:2585341] fixed: .HiraKakuInterface-W3
2020-06-20 01:26:52.274956+0200 fonttest[40345:2585341] latin: 1