Some of what I’m saying applies to all Congo travel (visas, in particular) but some of this will only be for Goma and the east.
Electric outlets, according to https://www.worldstandards.eu/electricity/plug-voltage-by-country/ should be of Types C, D, and E, but we found that some of the outlets were also Type G. Type G is the standard used in Uganda, and proximity to Uganda is no doubt responsible for this. I was glad I had brought all of my adapters, instead of just the ones I was sure I would need.
In order to come to Congo, you need to have a) a Letter of Invitation from a responsible person in the country, and b) a prise en charge, a letter stating that the person will be responsible for you and your behavior while in the country. The Letter of Invitation must be notarized in Kinshasa, even if the event is on the other side of the country, and your travel doesn’t take you through Kinshasa.
Make sure you bring copies of this letter when you come, as you may be required to show it to the immigration officials. The presence of your visa is not adequate as proof that you are permitted entry.
In Goma, French and Swahili are widely spoken. English is not commonly understood, and even those with relatively good English will often struggle to understand and make themselves understood. Lingala is reasonably well understood, but not as many people speak it.
It is recommended to have a typhoid and hepatitis A vaccine, and Yellow Fever is required. Although WHO now says that a Yellow Fever shot is good for life, it would be best to make sure that your vaccination is no older than 10 years, to avoid immigration problems. Malaria prevention is important, and more information about what works is available at the CDC website. I use Doxycycline in these areas because I have no contra-indications, it’s cheap, and the side-effects are less than some other medicines.
After we arrived at Goma we learned that there is also a need for an exit visa to leave the country. It costs $50 cash, and is paid at the airport. (The fee is less if you are not flying on Ethiopian Airlines).
There are three cell providers in Goma: Airtel, Vodacom, and Orange. We found frequent total outages of Airtel while there. If I travel there again I will try Orange, who advertise 4G speeds.