Programmatically specifying a document language lets screen readers know how to properly pronounce your content.
Be sure to also identify sections where the language changes, such as quotes or foreign phrases. If your app is localized, the programatic identification should update when the language updates.
Most images should include an alternative description, for those who cannot see it. An alternate (also known as alt) description should succinctly describe the content of the image.
The W3C’s WAI Web Accessibility Tutorials provides an alt decision tree to help you determine what images need a description.
Checkboxes, radio buttons, and single and multiline text inputs all need to have a label programmatically associated with them. The label provides a description of what the input is for. Labels should be concise and describe what kind of input the form represents (ex:“Phone number”).