Internet & apps in Korea
SIM card at the airport, Naver instead of Google Maps, Papago for translations, and a warning about opening hours.
The apps you'll actually use in Korea are not the ones you use at home. Here's what worked for me.
Get a SIM card so you have 4G everywhere
Seoul is huge. Even if I'm quite good with directions, I really found it useful to have internet all the time.
I booked a SIM card in advance and picked it up at the airport. It also came with a metro card, which I had to charge with Korean won. It helped me avoid the long queue at the airport.
Don't use Google Maps - use Naver
Google Maps won't work properly in Seoul. The information isn't really updated there, so I recommend using Naver.
It's quite nice because you can create lists and bookmark specific places. It even tells you which exit to take or which part of the metro to get in.
The only issue with Naver is that it can be hard to look for something if you don't have the Korean name. I prepared my list of places to visit in Google Maps and then had to copy-paste the Korean name into Naver to bookmark it.
Use Papago for translations
Papago was honestly one of the most useful apps in Korea. Not many Koreans speak English, and even when you order on tablets, the English translation isn't always 100% clear.
I used Papago most of the time because you can easily take pictures and see the translations. I'd recommend downloading it before going, especially for menus, signs and small questions.
Be ready for unclear opening hours
Something very weird is that the opening hours of places are not always correct.
You can only really check by going there, or maybe by sending a message on KakaoTalk if the place has contact details.
This is also why I'd avoid planning the whole day around one specific café or restaurant without checking if it's still open.