for word 2007
right-click on the word and word 2007 should auto detect the language and synonyms.
in case this does not work (which would be rare), check to see if ms word 2007 has the dictionary settings correct. take not that the correct setting should NOT
be set to use main dictionary only.
also at times, word might be confused if the text you copied is from a website or a non-plain text document. i suggest that for these cases, try pasting the text into notepad have, then copy from notepad, and then paste to word. i personally use this method of copying text from websites and then pasting them into notepad then to word. this removes any special formatting and copies only the text.
hope this helps