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In the image example this would mean: Translate all texts from English to German. If you want to translate one one or two texts of the detail layout, you can uncheck all texts which should not be translated.
If you already have texts for your target language, it is possible that the new translation differs from the current texts. In this case a conflict solving modal window appears. In that window you can decide for each conflict pair if you want to keep the original text or replace it with the new translation.
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