Differences between languages
Phonetical differences are not the only difficulties that one encounters when learning a foreign language: there are also substantial differences in * Grammar (like noun inflections in Slavic languages or Hungarian/Finnish, different use of tenses in English and romance languages, different conjugation patterns in Arabic, etc.)
- Vocabulary - English is close to Germanic and Romance languages, but have little in common with Slavic languages and almost nothing in common with Arabic or Chinese
- Writing system - that might be an easy one when it comes to learning Cyrillic or Arabic script, but a substantial one when learning Chinese or Japanese.
Teaching a child
I hope that you are not discouraged by what I wrote above: children rather early learn to recognize the phonological system of the language spoken around them (that is they develop ear for the phonetic differences important for this language and.) It is even claimed that French and German babies cry differently. Thus, exposing a child from birth to a second or third language is very beneficial, if they want to study languages in future - just having one's ear capable to recognize differences between two or three languages is helpful for learning other ones. Important thing here is that this should be authentic sound - speaking German or French with American accept is not likely to do the trick.
Note that speaking a language around the child does not guarantee that they will know this language (i.e., popular attitude one parent - one language is largely a myth). The reason is that the child's exposure to different languages is likely to be different: if one parent speak to the child in French, but everyone else in school, family and street speaks English, they will likely speak only English, although they may understand some basic French and have their ear tuned to French sounds (i.e., they will acquire some passive knowledge of language). The situation is different, if one language is spoken at home and another outside or if the family often spends vacations in a country where the language is spoken. One crucial element is the language spoken by parents - if the French-speaking parent speaks English to their spouse (e.g., because the latter doesn't know French) it would be hard to Force child speak French.
Thus, teaching more than one or two second languages is practically difficult - E.g., one could imagine a situation with three languages: one parent speaks French, the other German, but they live in the US. Provided that they often see their French and German family this could work. I do know people who grew up speaking more than three languages, but they usually come from countries where multiple languages are spoken around (like India).
TL;DR: It is beneficial to expose a child early on to sounds of a foreign language. More than two-three languages is usually impractical. They however will have their ear tuned to appreciate phonetical differences beyond their mother tongue, which will facilitate learning languages in future.