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.