Hello,
yes, it is possible this error appears when JoomSEF is activated, because it loads the translation file to be able to use it for URL localizations. (earlier then it would be loaded normally, when headers were already sent, so the error does not show up)
Also, it is possible, the extra character is not visible in the text editor you are using. Try to view the file in other editor - the best would be some basic commandline editor or notepad. (the charater maybe in certain encoding and more advanced text editors may render it invisible) You may also delete the whole part <?php and then write it again resave -- make sure the encoding will be Latin1 (or ASCII).
However, based on the message "output started at /home/..path../components/com_mtree/language/english.php:1" I am pretty sure this is the cause of the error - I have already seen such earlier.