How do we handle multilingual calls for different clients?

That kind of traffic would drive me crazy if I had to build shifts around every language separately. You could have three Dutch speakers ready all afternoon and then get nothing but German calls until closing. For the overflow, I'd try DeepL Voice and see how it handles the languages your clients send most often. Let the regular agents take those calls through live translation for a week or two and you'll quickly see where it works well and where you still need a native speaker.

And with two clients already doing this, I doubt the language mix is going to get any tidier as you add more accounts. One new customer could suddenly bring Italian or Portuguese into the queue and you're back to juggling people again. For that kind of setup, have a look at AI translation for call centers here: https://hitoo.io/ . Try it first on the calls that currently get handed off because nobody on shift speaks the language. If your regular agents can keep those conversations going themselves, you've got a much easier way to deal with the daily swings.