An incomplete and growing list of mistakes that I'd fix if I had a time machine:
Make all languages be written vertically, and stack their lines left-to-right. Everyone being on one writing direction is A+, and vertical writing gives more time for the ink to dry before your hand has to touch it when you're left-handed.
Figure out a universal script that everyone can use. Doesn't have to be too complicated; tons of languages with different sounds use the Latin alphabet fine. I'm kinda in love with the Korean script, tho - it's alphabetic, but is written like it's ideographic, with the letters arranged into syllable-blocks in a standard way. Something like that would be good.
Putting everyone on a single language would be good too, but probably really hard to maintain. Just making sure everyone's writing in a mutually intelligible way is a good start.
Switch everyone to base 6 counting. It's got some great reasons behind it!
- Each of your hands is a single base-6 digit (can represent the values 0-5), so you could count to 35 instead of just 10.
- It's got good divisibility - /2 and /3 (and /6 of course) are just "check the last digit", /4 and /9 are "check the last two digits", /5 is "sum the digits and check if they're /5" (same as /9 in base 10), /7 is "alternately add/subtract the digits and check if they're /7" (same as /11 in base 10).
- The multiplication table is really trivial, almost insultingly so. Similar to the divisibility, multiplying by 2 and 3 are now super easy (like multiplying by 2 and 5 are in base 10).
* | 1| 2| 3| 4| 5| 10| ---------------------- 1| 1| 2| 3| 4| 5| 10| 2| 2| 4|10|12|14| 20| 3| 3|10|13|20|23| 30| 4| 4|12|20|24|32| 40| 5| 5|14|23|32|41| 50| 10|10|20|30|40|50|100|
Numbers in base 6 are about 50% longer than in base 10, which isn't a huge loss. We can compensate for it by making sure the names for the digits are single-syllable, as that boosts your ability to remember strings of digits. Grouping digits into sets of 3 would also feel more "natural" - we might even be able to go up to sets of 6 instead.
I guess when talking about computers, we'd generally use octal (like we use hexadecimal in base 10) - 012345TE. Octal is a lot easier to learn than hex, too.