Internal links are translated automatically by TranslatePress. All the page or post links within your site are translated to their version of the currently displayed language.
Custom internal links that are added directly to the HTML (without using PHP functions to construct them) are those added in the post content for example. These links can also be automatically translated by TP if the option Force language in custom links is set to Yes. This option is active by default and can be changed in Settings -> TranslatePress -> General tab.
What』s left are links to internal media such as images, documents or archive files, and external links. These can be translated manually from the Translation Editor interface.
Simply hover a link, click the button and translate the link. For each language you can either place the translated link or select a resource using Add Media button. This allows you to use the WordPress Media library to select an existing image, PDF or other files, or upload a new file.
In this screenshot we have translated the external link of the Wikipedia article in English, to the French version of the Wikipedia article.
Basically we have translated the href of that anchor to point to another url.