I'm sure that for people from other countries, this custom/culture must seem even more peculiar than it does for me.
ご質問ありがとうございます。
"I'm sure that for people from other countries,"＝「他の国々の人たちによっては～に違いないと思う、」
"this custom/culture must seem even more peculiar"＝「この習慣・文化はもっと不思議に見える」
"than it does for me."＝「私にとってよりも。」
ご参考までに。
Some of our customs must seem even more strange to people from foreign countries.
I bet people in other countries must think they are even more odd than we do.
この場合、次のような言い方ができます。
ーSome of our customs must seem even more strange to people from foreign countries.
「私たちの文化のいくつかは、外国人からしたらもっと不思議に思えるに違いない」
seem even more strange to people from foreign countries で「外国人にしたらもっと不思議に思える」
ーI bet people in other countries must think they are even more odd than we do.
「外国人はそれらを私たち（日本人）が思う以上に不思議に思うだろうね」
must think they are even more odd で「それらをもっと不思議に思うに違いない」
ご参考まで！
It must be even weirder to people from other countries.
「海外の人にとっては、もっと不思議だよね」
- It must be even weirder to people from other countries.
- For people overseas, it must be even stranger.
- If it seems odd to us it must be even more so for those from other countries.
アメリカ合衆国 
カナダ 
