1. A computer-implemented method, comprising: receiving, at a client device, a document for a web page, the document comprising document text in a first language to be displayed on the client device; displaying, at the client device, the web page in web browser software executing at the client device, wherein the web page includes the document text of the document; identifying, at the client device, a second language to which the document text may be translated; determining, at the client device, whether the second language is different from the first language; displaying, at the client device, a translation header as part the web page above a portion of the document text to be translated when the second language is determined to be different from the first language, the translation header including (i) a selectable option to translate the portion of the document text from the first language to the second language and (ii) text in the second language describing the selectable option; and in response to a selection of the selectable option: identifying, at the client device, the portion of the document text to be translated; sending a request from the client device to a translation server to translate the portion of the document text from the first language to the second language; in response to the request, receiving at the client device translation data including a corresponding translated portion of the document text in the second language; and displaying, at the client device, the translated portion of the document text of the document as part of the web page.
6. The method of claim 1 , wherein: the document is represented by a data structure, the data structure including text nodes, each text node including respective text of the document text; sending a request from the client device to a translation server to translate the portion of the document text comprises sending data including the text in the text nodes, as distinct objects, to the translation server; and displaying the translated portion of the document text in the document comprises: replacing the text in the first language in the text nodes with corresponding translated text in the second language; and rendering the data structure having the translated text in the text nodes.