Since now i used wp-forecast with this trick:
i edited "en_US.po" and translated the strings i wanted in greek language.
After updating to lated version this trick stopped working. So i did the same with the file "wp-forecast_en_US-en_US.po". It didn't worked.
Then i tried this. From file "wp-forecast.pot" i created a new .po file named "wp-forecast_el_EL-el_EL.po" which i transferred to directory "/lang" (of course and the "wp-forecast_el_EL-el_EL.mo" file). Also i edited "wp-forecast-admin.php" and changed a line for "german" to this:
<option value="el_EL" <?php if ($av['wpf_language']=="el_EL") echo "selected=\"selected\""?>>greek</option>
From admin panel i changed settings for language to greek but the widget on my site it still showing english language.
Do you have any ideas that could help me?