Calcite is the name of a branded visual design system for Esri.
Calcite Components are a set of reusable web components built using Stencil.js. With Calcite Components, you can quickly build on-brand, lightweight, and accessible web applications.
They were created for developers to be used in any framework and across any part of a web site or web application. The set is meant to be flexible enough so that you can accomplish 80% of your web application using them. The codebase is well supported, steadily maintained, and always being improved.
Web components are reusable custom elements with their functionality encapsulated away from the rest of your code. They are built on Web Component standards, will work across modern browsers, and can be used with any JavaScript library or framework that works with HTML.
All of our components that provide strings in the user interface allow those strings to be set via a property. This allows you to define the string for the locale you would like. Your application can handle internationalization as it sees fit.