When “who” is the object of the preposition, as in this case, it becomes “whom”; granted, this is by now vestigial and often ignored in informal conversation. You’ll often hear people say things like, “Who should I give this to?” It would be correct to say “Whom should I give this to?” and misguided fussbudgets will insist you render it as “To whom should I give this?” But almost no one bothers with that these days. Note that reversing the word order makes the incorrect grammar stand out: “I should give this to who?” That’s because there is now a direct apposition with the preposition and its object. Most careful speakers will use “to whom” in that context.

You can remember when to use “who/whom” by substituting “he/him” in the sentence. You wouldn’t say “I’m doing the project with he,” you would say “I’m doing the project with him.” So it’s obvious that whom is the pronoun you would use here, not who.

A further word about German/English prepositions. In German some prepositions can be dative or accusative, depending on whether they indicate motion or placement towards or up to a location. This not the case (no pun intended) in English. In English, the object of the preposition always takes the “prepositional” case. Note that there are not nearly as many inflectional changes or pronoun substitutions in English as in German. The point is, German is not necessarily useful for analogizing English constructions.