If a language text has not been translated yet, or has been translated incorrectly, please correct the relevant language file and send it to us so that we can include it in the next release. This will save both you and other people time in the future.
If a translation is not really wrong, but you prefer a different translation for your site, the best solution is to use the Joomla Language Manager to create translation overrides. To create a new override, go to Extensions, Languages, Overrides, and click New. Search for the text you want to change, then click on the result you want to override. Enter your preferred text and click Save. Overrides created like this are always preserved, even if you re-install or upgrade the software.