Just replacing and with or removes the ambiguity:
Our cakes are very popular among foreign visitors.
Some of our customers speak Spanish, English, or German.
"Or" would be interpreted in this case to mean "at least one of …".
However, that still doesn't make these two sentences well constructed. I have a hard time figuring out what connection you are trying to draw between linguistic ability and food preferences. I suggest combining all of the facts into one statement:
Our cakes are especially popular among our Spanish-, English-, and German-speaking customers.
That makes it clearer that you are categorizing your customers by inferring their place of origin through their language. Or, if you wish to to use a simpler and less specific expression, you could, in some contexts, say "western" to mean roughly European / Australian / North American / South American. (There's no point in being exact, since you are just expressing general preferences anyway.)
Our cakes are especially popular with our western customers.
2In formal logic, the and would mean that some of your visitors speak all three languages. Simply use or and it means that some of your customers speak one or more of those languages. – Polygnome – 2016-05-30T18:27:08.780
6"Our cakes are .... Some of them speak ...." Yeah, talking cakes that speak three languages would certainly be popular for foreign visitors! ;-) – alephzero – 2016-05-30T18:34:47.507
1When repeating like this, you can shorten the third one if you have to: Some of our customers speak Spanish, some speak English, and some, German. – J.R. – 2016-05-30T22:46:26.430
It is guaranteed that people will get confused. – Ross Millikan – 2016-05-31T03:38:33.490