Lingoes is an easy and intuitive dictionary and text translation software, It offers lookup dictionaries, full text translation, capture word on screen, translate selected text and pronunciation of words in over 80 languages.
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Pro Appendices System (conversions and so on)
Pretty unique and nice feature: it includes Currency Converter, International Dialing Codes, International Time Zone Converter, Weights & Measures Converter and more.
Pro Ability to translate any word under cursor
by pressing ctrl + right click.
it is not stated, if this word may be a part of system's UI
Pro Text translation, not just words/phrases
The online translation service offered by Lingoes integrates the most advanced text translation engines in the world, including Systran, Promt, Cross, Yahoo, Google and Altavista, etc, which makes text translation unbelievably easy. You can freely choose these engines for your translation and compare the different results generated by different engines to help you understand the texts in languages which you are not familiar with.
Con Doesn't support XDXF format.
Has a collection of dictionaries on its site, but doesn't support many formats (if any).