Mona Baker, Egyptian professor of translation studies and Director of the Centre for Translation, declares in her book ‘In Other Words: A Coursebook on Translation’ four solving strategies to translate idioms:
1) using an idiom of similar meaning and form.
2) using an idiom of similar meaning but dissmiliar form.
3) translation by paraphrase.
4) translation by omission.
In other words, if you want to translate an Idiom you:
* try to find an idiom in the target language (is the language you're translating into) which uses the same words, the same structure and has the same exact meaning.
* try to find an idiom in your language which uses different words but has the same structure and the same meaning.
* try to find an idiom in your language that has different words, different structure but the same exact meaning.
* try to find an idiom in your language that has different words, different structure and slighlty different meaning and complete it with a short explanation.
Don't try to use an automatic translation tool because the results will be very funny and sometimes embarrassing.
For example:
In English language we have the idiom 'once in a blue moon'. To describe the same idiom in Polish it's tempting to translate this expression literally into: 'raz na niebieski księżyc'. And now we try to tell a Polish person that something happes 'raz na niebieski księżyc'
-> Chodzimy do restauracji raz na niebieski księżyc. <?>
This expression usually makes Polish people feel embarrassed. So, a Polish idiom that means exactly the same as the English idiom is 'raz na ruski rok', which literally in English would be 'once in a Russian year'. The English people don't understand it.
In Italian language 'once in a blue moon' means 'ad ogni morte di Papa' (literally: a every dead of pope).