Should slides and your talk be in the same language? Ideally, yes. Visuals and the audio track are in perfect sync.

But I think for most audiences in Western economies, "business English" slides that support a talk delivered in a local language work perfectly OK. "Business English" is what I call the English that is spoken by most non-native English speakers. A very narrow vocabulary of English that enables you to express most common business concepts.

For some audiences having your slides might give you that added international appeal (a startup raising money across Europe for example, or here in Israel, where high tech slides designed in Hebrew would look really weird).

Slides in English raise the challenge for the presenter though. If you were planning on reading bullet points of the slides, it sounds boring in English, it sounds really awkward when you are live translating from English into your native language. Either things go really slow, or the translation sounds really funny, or - most likely - both.

As always, there are exceptions. Some highly conservative financial institutions have complicated investment approval processes where decks get forwarded/discussed without you being there. If your deck is primary for reading, then consider translating the whole thing.

Be aware that languages can create technical challenges as well if people do not have the right fonts installed on their computers, and mobile devices create additional problems. Always send PDFs.

I have done many of these types of projects for presentations aimed at local Israeli institutional investors. I would start with an English design (but laid out right-centered, graphs flow from right to left), the client would translate (challenge 1: Hebrew, challenge 2: business/science-specific jargon in Hebrew), and would clean up afterwards with a 50% understanding of what's inside the text boxes. 

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