To build your own menu bar app, I would recommend trying out xbar[1].
You can write "apps" in any language. The output of your program is used to construct the menu bar app, so if your program outputs this:
Hello
---
Open HN | href=https://news.ycombinator.com/
Then it will create a menu bar entry titled "Hello" with a single element in its drop-down menu, which will open Hacker News when clicked. You place your program in a specific directory for xbar to run, and it can be configured to run at a given frequency, e.g. once a minute to report the results of an API call or some measurement from your system, etc. Up to you to build the menu bar app you want.