Choose "View->Interface Language->Language Name" from the menu to switch InstantGet into localized interface of the selected language. Localization will be applied immediately. No restarting is needed.
You are always welcome to make contribution to InstantGet localization. You can either create a new language file or improve an existing language file.
After editing the language files, you can email the updated language file to us so that we can include the changes in the next release. Your changes about un-translated entries will always be included. However, to modify an entry that have been translated, you need check whether a language file has been assigned to a chief translator. If it has, you need contact the chief translator by email and suggest possible improvements.
If you are interested in being a chief translator for a particular language, please don't hesitate to contact us. Being a chief translator is a volunteer job making InstantGet more friendly to non-English users.