You can manually exclude specific pages from being translated by selecting the One file per page option when creating the translation ZIP, then manually deleting the translation files of the pages you don't want to translate.

In the translation ZIP, the translation files for the pages are named as follows:

<pageID>_<pageTitleInDefaultLanguage>_<defaultLanguageKey>_<targetLanguageKey>

When importing the file back into Confluence after translation, the deleted pages will not be imported, and will only be available in the default language (and any other languages they have been translated into).