Googling Google posts about a hidden Gmail operator that lets you restrict your messages to a certain language. For example, if you search for lang:pt or lang:Portuguese or language:Portuguese, you'll find some of the messages that contain Portuguese text. You can combine the lang: operator with keywords or other operators. The language detection is not perfect, so not all the messages are labeled correctly.
Gmail could provide an option to automatically translate messages written in a language you don't know and add many other useful features from web search (spelling suggestions, query expansion, search refinements).