Greasy Fork is available in English.

Writing user scripts requires knowledge of JavaScript. The tutorials on the GreaseSpot Wiki should give you a running start on JavaScript both in general and as it relates to user scripts. Inspecting others' scripts can be helpful. You may be interested in Greasy Fork's rules for posted scripts and Script meta keys read by Greasy Fork. If you want help developing your script, feel free to ask in the forum.

To post on Greasy Fork, you will need to create an account. From your user page, follow the link to post a new script.