The rule of thumb for apps is you want to maximize the ability to use these apps offline in case you are in a remote, unserviced area or don't want to splurge on an international SIM card / roaming plan. Before you go, here are some handy apps to keep in a travel folder.
When heading to a foreign country, Google Translate is a must. Ideally, you should download the language you want ahead of time so you can use it offline. Same goes for Google Maps. You can select an area you want to save and navigate without internet connectivity.
While you can opt to buy a local SIM card, assuming you've got an unlocked phone, WhatsApp, Facebook Messenger, and Viber have made it easy to make international calls for free. Just remember to add your contacts in before leaving and hope your hotel Wi-Fi is strong enough to support a quick chat.
Many cities around the world offer local ride-sharing or taxi-hailing apps, so it's worth checking if that's something you're willing to splurge on and download in advance. Having this tied to your credit card will eliminate to need to have cash handy first thing when you're leaving the airport and getting a ride to your hotel. If you want to have some cash handy, however, you are more likely to get better rates exchanging it in advance with your local bank than to do so at airports.
Major credit card companies like Visa, Mastercard, and American Express offer apps that can help you find local ATMs, prepare a mobile wallet, or convert currencies. Just be sure to notify your banks if you plan on using the cards abroad so you're not unexpectedly locked out.
In my opinion, these apps are the bare minimum you should have before taking off, but for specific interests like food spotting or finding locals to hang out with, BonAppetour has a great list for a variety of needs.