You find the translation of the IDE strings in the files lazaruside.<code>.po and debuggerstrconst.<code>.po in folder languages of your lazarus installation. The LCL strings are in file lclstrconsts.<code>.po in folder lcl/languages. And many components have their translations in their own directory (e.g. TAChart in components/tachart/languages/tachartstrconsts.<code>.po). <code>, here, is the language code of the translation, e.g. it für Italian.
I usually do not touch the original po files in an editor because it is very easy to break something. Better to use a translation tool, such as POEdit. The basic version is free (there is also a commercial "pro" version). After installation of POEdit, doubleclick on one of these language files in the expllorer to open it. POEdit displays a table with the original English text in the left column and with the so-far translated strings in the right colum. Empty cells indicate a missing translation. Warning icons in the left column indicate that something is missing in the translated text. In the screenshot (which is for the German POEdit, sorry) the first item has the issue that the original string ends with a colon and a space which are missing in the translated text. Keep a eye on such inconsistencies, fixing them makes the translation look a lot more professional.