この街は坂道が多いので電動自転車で移動する人が多いって英語でなんて言うの？

So long as you mention the key information (the town, the hills and the electric bikes), you should be fine. There are lots of different ways to phrase what you want to say. You could also expand your explanation: for example, ‘A lot of people in this town use electric bikes because there are so many hills and the bikes make it easier to cycle up them’ or ‘Electric bikes are very popular in this town because people use them to cycle up and down all the hills’.

slope は「坂道」hill は「小山」又は「坂」という意味です。
「街」は city や town でも大丈夫ですが、無難に this area （このエリア）など、this neighborhood （この近所、この地域）という言い方でも大丈夫です。
「電動自転車」は electric bicycle 又は electric bike です。

1)This town has many hills so most people use electric bicycles.
（この街は坂道が多いのでほとんどの人は電動自転車を使っています）

２）This area is hilly so people use electric bikes to get around.
（このエリアは坂が多いので人々は電動自転車で行動します）
hilly = 小山や坂が多い

どうぞご参考に！

Examples:

There are so many hills in this town, so a lot of people use electric bikes to conserve energy.
So many people use electric bikes here, because this town is so hilly.
It is tough riding bikes in this town because there are so many hills, that is why so many people use electric bikes.

When talking about a certain (this) town, youwant to explain that it has many hills so many people use electric bicycles. In that case one of the above suggestions may be appropriate for your scenario.

Ubiquitous might apply here... as in present, appearing, or found everywhere!
Hills are ubiquitous in our town, so everyone uses electric bicycles to get around.
Simple " pedal power" is not going to be sufficient...
Or in a more gentle fashion, the undulating terrain makes it hard for cyclists, therefore people need electric power to help them get around our town!

This town has so many hills that many people choose to use electric bicycles. By saying this town, it makes it clear that you are referring to the place where you currently are. Hills is a way of expressing many inclines and declines in a certain area. So is a way to connect the first thought being a cause, to the result being that many choose to use electric bicycles.
Because there are so many hills, many people find it easier to use electric bicycles. By starting with because you are implying that the first part of the sentence is the reason for what follows. The reason being that there are so many hills, so it is easier to use electric bicycles.
With all of the changes in elevation is a way to refer to there being many hills going up and down in the town. Again, because of this, it is easier to use an electric bicycle.

If you were talking about the fact that a certain town has many hills so it is common for people to use electric bicycles, you could express this by saying: "This town has many hills so many people use electric bicycles" or "Electric bicycles are popular in this town as it has many hills".

In the first two examples we can use the term, "a lot of hills," or, "very hilly," to discuss that there are a lot of hills within the town or city. In addition we can also move the sentence structure around and still maintain the same meaning.