URL aliases not working for content not in default language

I'm migrating a website which is almost exclusively in English, but has some content in other languages (translation of important documents, etc), so it's not truly a multilingual website, only an English one with some pages in other languages.

I'm using the Locale and i18n modules and set them up to use no language negotiation system (no path prefix or subdomain), and to display "All content. No language conditions apply.". This way, a node in another language than English is always visible, and shares the same URL scheme.

My problem is that although the "non-English" node is visible, and everyone is able to access it using Drupal's internal URL (node/123), the custom URL (like "content/page-title") is not working, returning a "Page not found" error message.

Basically, if one's default language is set to English :
- one create a node in "Language neutral" -> the URL alias works
- one create a node in English -> the URL alias works
- one create a node in English then translate it to another language -> the URL alias works only for the English version
- one create a node in another language than English and with no translation, or one is trying to access a translation in another language than English -> the URL alias doesn't work

URL aliases that don't work are still displayed in the URL alias list (admin/build/path/list) and in the url_alias database table.

Is it something that can be a bug in the Path module, or a problem more related to the way the i18n or Locale modules are restricting access to localized content?