Yes you will need a valid and up to date passport for entry into the Dominican Republic. You will also need to fill out an E-ticket through https://eticket.migracion.gob.do/ in order to check in and board your flight. to and from DR.
Santo Domingo is a very modern city and like most cities there are good areas and bad areas. Staying in the Colonial Zone is very safe and there is a lot of police constantly in the area, day and night. With that said excercise normal precautions when traveling in a foreign city and be aware of your surroundings.
Download Google Translate App and then go to the account icon, then press downloaded languages, then select Spanish to download the Spanish language on your phone. This will work with or without phone service. Here are some useful cheatsheets to get by: https://independentatlas.com/ultimate-language-cheat-sheet-guides/ultimate-spanish-language-cheat-sheet-guide/ https://www.theintrepidguide.com/wp-content/uploads/2024/05/The-Intrepid-Guide-Spanish-Travel-Phrases-Cheat-Sheet.pdf https://www.dummies.com/article/academics-the-arts/language-language-arts/learning-languages/spanish/spanish-for-dummies-cheat-sheet-208766/ https://letsspeakspanish.com/blog/spanish-for-travel-vocabulary/#hotel
Most major phone carriers have international service. You will need to call your provider and activate the international service for the duration of your stay. For example, Verizon has international service for $12 each day per line if it is activated as part of your plan. The service will operate off of the local Claro network and the service is pretty decent everywhere. You can also get a sim card from from the Claro store after clearing through customs. The people at the store don't speak much english but if you point to the signs in english, then they will understand and set up your phone for you. It is about $8 to $10 for 10 days of phone service.