Many themes, plugins, and even WordPress Core use English as a default language. Because of this, if you use a different default language for your site, you will have strings belonging to different default languages. These strings may get categorized in the wrong language. In order to translate them successfully, you may need to correct the source language of some of your strings.
String Translation gives you the option to change the source language of both individual strings and entire domains. Please follow our documentation about changing the source language of strings for more information.