How can I start translating through the translation portal?

Follow the processes as below:

1) Create an account on Crowdin and join the translation team (see instructions here).
2) Once you have an account, go to the Translation Portal and select your language.
3) Choose one from other content or platform the menu on the left.
4) Select a subject, for example, Early Math, and select an item (see goals and milestones here) on the right that needs to be translated. You can uncheck untranslated on the top to see all contents.

5) This will bring you to a Crowdin login window. Select your language and log in. You’ll be required to join the team of your language.
6) When you see the Crowdin In-Context tool as below, you are all set to translate! Read here if you don't know how to use it.