In OpenOffice, it is possible to choose:
Tools > Language > For Selection / For Paragraph / For all Text
In Evolution, when creating a new message, it is possible to select more than one language at the same time:
Edit > Current Languages (you should be able to select more than one)
These are just two examples. I'm not able to do it in Firefox or Pidgin, so I think it depends on the application that you use.
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