openwithrosetta - a tool to check the box "Open with Rosetta" on Apple Silicon Big Sur Macs

main.m

//
//  main.m
//  openwithrosetta
//
//  Created by Simon Andersen on 13/09/2021.
//

#import <Foundation/Foundation.h>

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        // insert code here...
        NSMutableDictionary *lsDict;
        NSString *appPath;
        NSString *bundleIdentifier;
        NSData *bookmark;
        NSString *lsPath = [@"~/Library/Preferences/com.apple.LaunchServices/com.apple.LaunchServices.plist" stringByExpandingTildeInPath];
        NSArray *arguments = [[NSProcessInfo processInfo] arguments];
        if ([arguments count] == 2) {
            appPath = arguments[1];
            if ([[NSFileManager defaultManager] fileExistsAtPath:appPath]) {
                lsDict = [[NSMutableDictionary alloc] initWithContentsOfURL:[NSURL fileURLWithPath:lsPath]];
                if (lsDict == nil) {
                    lsDict = [[NSMutableDictionary alloc] init];
                }
                if ([lsDict objectForKey:@"Architectures for arm64"] == nil){
                    [lsDict setObject:[NSMutableDictionary dictionary] forKey:@"Architectures for arm64"];
                }
                NSMutableDictionary *arm64dict =[lsDict objectForKey:@"Architectures for arm64"];
                bundleIdentifier = [[NSBundle bundleWithPath:appPath] bundleIdentifier];
                bookmark = [[NSURL fileURLWithPath:appPath] bookmarkDataWithOptions:NSURLBookmarkCreationSuitableForBookmarkFile  includingResourceValuesForKeys:nil relativeToURL:nil error:nil];
                if ([arm64dict objectForKey:bundleIdentifier] == nil) {
                    [arm64dict setObject:[NSMutableArray arrayWithObjects:bookmark,@"x86_64", nil] forKey:bundleIdentifier];
                }
                else {
                    [ (NSMutableArray *)[arm64dict objectForKey:bundleIdentifier] addObjectsFromArray:[NSArray arrayWithObjects: bookmark,@"x86_64",nil]];
                }
                [lsDict setObject:arm64dict forKey:@"Architectures for arm64"];
                [lsDict writeToFile:lsPath atomically:YES];
                NSTask *killalllsd = [NSTask launchedTaskWithLaunchPath:@"/usr/bin/killall" arguments:[NSArray arrayWithObjects:@"lsd", nil]];
                [killalllsd waitUntilExit];
                
            }
            else {
                NSLog(@"Invalid path: %@",arguments[1]);
                return 1;
            }
        }
        else {
            NSLog(@"Not enough arguments");
            return 1;
        }
        
    }
    return 0;
}

Made a Py-Objc version at https://gist.github.com/WardsParadox/d6a8fbe0c0d1f100b5574188e4bd627d