There are 2 options within the Languages Menu.
The first is the app language - this affects the text in the menu's and headers of the app - the options for languages in this section are quite limited, but the text in these areas tends to be supported by pictures to show what the item is.
The second is the translated language of text content sent to you by us, such as letters, forms, news items, and messages. This option covers the 259 languages supported by Google.
To change the language in either option click on the relevant button.