Politicus, your first solution would be fine if I had only a few sentences, but to find the good sentence will be a hard task if I have hundreds or thoushands of sentences in the clipboard history (database).
As for the other solution, I already have something quite good.. that uses Alfred to launch mdfind and grep to search for the selected text in a folder (I have millions of sentences in that folder, in thousands of tab delimited txt files). The search syntax is:
mdfind -onlyin /Users/.../folder containing hundred of small tab delimited texts \"{query}\" | xargs grep -i -h --max-count=50 --color "{query}"
In 2 or 3 seconds I get all the sentences that contain the selected string (full sentence or a few words), and the searched string is colored so I can find it easily in the results (the string can be in the middle of a long paragraph). The search is triggered with a hot key, with the "Selection in OS X" argument selected.
In can also add my news translated sentences to a txt file, with another hot key, with the following syntax:
echo "{query}" >>/Users/...path to my tab delimited.txt file && osascript -e 'tell application "Terminal" to close window 1'
The first part adds my selected text at the end of the file, and the second part closes the terminal's window.
(I have another one to delete a string when I realise that it contains a mistake, but it's becoming a bit off topic ; )
The only problem is that it uses mdfind, and mdfind doesn't index all the words. Frustrating. That's why I started using the clipboard history, since it uses sqlite and probably makes a better indexing job than mdfind (Spotlight). For the moment I copy the text and launch the clipboard search. Segments are also quite easy to del from the history (fn delete). I just wish I had the possibility to launch the clipboard search with the selected text. Maybe I should make a feature request for Launching Clipboard Search with selected text... after all it's similar to Show Alfred with the selected text...