Hello Chevere33,
It seems like you are confusing "어딘가" with "어디인가". The former translates to "somewhere", the latter "where" in an interrogative sense. The sentence also seems to end rather abruptly, but since it is not clarified how and where the sentence is used, I will refrain from suggesting an alternative in this case. If correctly written and spaced, it goes "누구나 어딘가에서는 외국인", which is simply saying "everyone is a foreigner somewhere (other than their home country)", and though the message is quite subtle, personally, I was able to understand what the sentence was really trying to convey, and so do other people I am sure. Then again, that's just me, and a teensy, tiny bit more elaboration can't hurt, as the underlying message could be interpreted differently (eg. homesickness, affinity to one's native culture, etc.). It could definitely be argued that the phrase itself may be suggesting an entire different idea. Say for example, "everyone outside the comfort of their own native culture/environment is a foreigner, so accepting, respecting, and learning about other's cultures with an open heart is important..." so on and so forth can totally be possible. I see why not. It is better to be safe and sure than sorry, so clarifying what you are trying to say right after might just be better. After all, the phrase seems to end more or less on an abrupt and incomplete note, as mentioned.
Oh and just a side note, "외국" refers to a foreign country, abroad. "외국인" is a foreigner. Hope this helps.