SharpDevelop uses English as the default lanugage. You can change the UI language as follows:
Select the Tools->Options menu item. This opens the Options dialog box shown below:
To select a different language, click on the flag icon corresponding to your preferred language. Your choice is reflected in UI language set to:, next to the "Current UI language".
Click OK to close the Options dialog box.
Close SharpDevelop and re-open it (language changes are not reflected immediately, a restart is necessary)