Mainland European Languages for US vs European UMM/MF PE Firms in London?

I've heard online that knowing a mainland European language is almost a requirement to transition to London PE firms from IB. But, I am wondering if this requirement only applies to the European PE firms or if it also applies to the US UMM/MF PE firms in London as well. Also, regarding the topic of languages, I am currently have a B2 level of fluency in French and I have been working on bringing it to a C1 level. French is the only European language I know apart from English obviously. Would C1 fluency be of any benefit to me during recruiting or do PE firms always look for native speakers?