Practicing good listening is important in asking good questions to keep a conversation alive. When your listening to someone are you just waiting for your moment to share your story or are you really understanding what they are trying to convey/share. Granted i dont use these things at work when there is a goal to the chat but rather when its a group of people sitting around having dinner together. I dont have a specific phrase i use in particular to keep a conversation alive but rather some “strategies”.
Ask follow up questions to probe deeper into the subject. Be like your 5th grade English teacher reminding you about the question words. Who, what, when, where, why, and how. If they didnt get one of those then ask about it. (if it makes sense to)
Repeat back to them what they said, in my experience it often gives the person a confirmation that youre listening, interested, and a base for them to continue talking about said thing.
In your native language its much easier to let a conversation flow from one topic to the next but maybe since its in a foreign language with presumably someone who is fluent maybe steer to things you feel comfortably speaking quickly about. Music you like, food recommendations, Travel, Books, hobbies. I dont have any evidence but there are times i think people just get tired of someone speaking their language back at them so slowly. Hopefully your friends are patient. (And hopefully you are patient when someone is clearly a learner of your language trying their best)
Dont feel like there cant be a lull in the conversation either as you both search awkwardly for something to talk about. Find something either person is passionate about and youd be surprised how much you can just let them talk about that one thing. Get someone started and watch them go.