I work for a company whose product does that automatically, but doing it manually on SharePoint Online is the same as doing it on premise. You said that Arabic has been selected as an alternate language, I am presuming the site was created in English, that you are not using a content type hub, and that the Columns and Content Types were created in English.
- Change your language to Arabic (more on this below). In all other steps the interface will be in Arabic
- While in classic mode, not modern, click on Site Settings in the Settings menu, then Site Columns or Content Types
- For each one you want to localize, if the source column is a hyperlink then follow it. That means it is inherited from a higher level site
- Click on the column or content type you want to localize. Then if the name appears in English, change the name to its Arabic equivalent and save. Don't worry, the name change will only apply when your language is Arabic, the English name stays the same.
Changing your language using the user profile can take a few minutes to work and it doesn't always apply to all screens. Instead, you can remove all languages from your profile then wait for a few minutes, and then change the first language in your browser settings. That language change will be instantaneous as long as there are no languages in your user profile.