Your website header and footer are unique in the sense that they are always present on your website. If you're running a multi-language website, the content in the header and footer repeat across all your languages.
However, it is sometimes necessary to differentiate elements in the header or footer for the languages you offer. The most obvious example is text. If you have added text in your header and footer, you want to localize that so it's in German on the German version of your website and in French on your French version.
In order to accommodate those scenarios, the following modules in the Editor can be edited uniquely for each of your languages in header and footer:
All other modules cannot be customized independently for each language.
Implications - examples
There are a number of implications to having language-sensitive modules in the header and footer.
When you add a Text module in your header or footer, the Editor automatically adds it for all your languages with a placeholder text "Type text here". If you change this text on the German version of your website, it won't change on the French version until you input something on there yourself.
If you add an Image module to your header or footer for German language, the Editor will also show it on the French version of your website, unless you decide to change or remove it. If you remove it, the Editor will remember that action. In the odd case where you may subsequently have deleted French language from your website but then add it later on again, the Editor still remembers that you removed the image and will therefore show an empty Image module.
You've added a Button module in your header or footer and customized the label text in German and French, respectively. A few months later you delete the French version of your website as you thought you wouldn't need it any longer. However, a few weeks later you realize it was a mistake and re-create the French version of your website. The Editor remembers the French text labels you inputted before and displays them again.