Here's some of the material I used for a month long after school Math In English class.
This was for high level 3rd and 4th graders. The math level seemed  good for them. A bit on the easier side since I was focusing more on communicating math using english.

2 days- big numbers
2 days- +, -, x, / words
2 days- +, - word problems
2 days- x, / word problems
1 day- all word problems game
2 days- plane shape names
1 day- making patterns with blocks
1 day- Cartesian plane
1 day- four color theorem
2 days- symmetry
4 days- tessellation

ooppss.. and here's the cartesian plane powerpoint.

If anyone's interested I can explain more about what I did in a lesson.

BTW, on the subtraction slide:

"Subtract 10 from 5" is not the same as "10-5."

"10-5" is "subtract 5 from 10."

The best way for kids to learn another language is to learn other subjects in that language rather than treating the language purely as a subject. There has been talk about teaching math, science and English in English, which I would wildly support. Sadly, I think that's pretty far into the future.

Keep doing your thing and teach math in English!

Some schools in Korea already do this (have the students take classes in various subjects taught in English), but it's basically only at private schools (meaning a private school a student attends instead of a public school, not a hagwon).

OP, I'm just adding my game here as it meets the topic. I think mixing English with other subjects is great! Especially for math, they really need to know their numbers!

This is a math game I made for my super low level HS students. The template (from waygook) is a little childish, but it worked and I was in a bind the day I made it. The game actually turned out to be a hit!

No worries, I'm also at a tech HS. It is great to get some ready to go material for them. Enjoy!