Extendable's some templates (404.html, archive.html, index.html, etc) strings are not internationalized, and some string translations are not available at https://translate.wordpress.org/projects/wp-themes/extendable/. We could add translations for these languages, as listed here: languages for now.
Here are some examples of how translation strings appear when WP language setting sets to the Dutch language.


This is only for templates. For template parts such as headers and footers, we will address them in a separate issue, as Extendify might require tweaks for headers and footers.