There's two questions here:

• What to do when someone speaks fast: If you don't understand you will have to speak up. However, do not interrupt them (e.g. during case prompt) and do not directly ask them to be slower. Instead, when it happens, do mention that it was a little fast for you as a non-native speaker and ask whether they could please repeat. At this point, a good interviewer will make an effort to slow down and make themselves understandable.
• At which pace to speak yourself: Whatever you feel comfortable with. No point in accelerating and starting to ramble because your brain can't keep up. Slowing down speech can be an effective tool for case ownership in combination with good top-down communication, especially if your counterpart is fast-paced. 

Hope this helps a bit! Best of luck!

Hi there,

If this is on the listening side, the best thing you can do is train your ear. Listen to audios/podcasts, watch movies etc.

If it's about speaking as fast as them, don't try! Speak at the pace that works for you. They'll understand.

Hello!

Two different things here: 

• The pace at which they speak: which you cannot control, all you can do is train yourself to this type of speakers. Do so with the classical podcasts and listening exercises that are everywhere. 
• The pace at which you speak: that you can always regulate, and that should be fine unless you sleak too slow

Hope it helps!

Cheers, 

Clara

Please understand that if you find hard to follow what people are saying in a given country, you may not be ready to perform work in that country. So you have to really make an effort to increase your language skills. This is important not just for your interview, but for your day 1 on the job.