TYPO3 Core

When the current page is a translated page, $mountedPage = $pageRepository->getPage($mountedPage); can't create a language overlay.

So at the end of PageRepository::getPage the call $result = $this->getPageOverlay($row); returns the same record even if the target language is different to the page overlay since the translated page itself is not translated.

That's why it works in the default language. There is the given $row the page in the original language and it can create a translation for the given page.

I'm sure this is not the right way but that's how I mad it work.

I added two XClasses for my project.

I couldn't find a way to get the languageAspect of the target page in the method resolveMountPointParameterIntoPageSlug. That's why I added one XClass only to add the method getLanguageAspect to the PageRepository.

\TYPO3\CMS\Frontend\Page\PageRepository

    /**
     * @return \TYPO3\CMS\Core\Context\AspectInterface
     * @throws \TYPO3\CMS\Core\Context\Exception\AspectNotFoundException
     */
    public function getLanguageAspect()
    {
        return $this->context->getAspect('language');
    }

Then I modified the method resolveMountPointParameterIntoPageSlug in the following way. There is probably a better way but...

                $mountedPage = $pageRepository->getPage($mountedPage, false);
                // XXX>
                /** @var \TYPO3\CMS\Core\Context\LanguageAspect $langAspect */
                $langAspect = $pageRepository->getLanguageAspect();
                if(($langAspect->getId() === 0 || !$pageRepository->isPageSuitableForLanguage($mountedPage, $langAspect)) &&
                    $mountedPage["sys_language_uid"] > 0 &&
                    $mountedPage["l10n_parent"] > 0
                ) {
                    $mountedPage = $pageRepository->getPage($mountedPage["l10n_parent"]);
                }
                // <XXX

                $mountRoot = $pageRepository->getPage($mountRoot, false);

Then the URLs are right in all cases. At first I only tried isPageSuitableForLanguage but this only worked if the languageId is not 0.

I'm not sure but maybe this should be solved in the pages repository.

I had the same problem here. But when I patched my code as Steffen did, everything looked fine on the first localization. But the second localization failed:

/en/page/subpage/
/sv/sida/delsida/
/no/page/under/

SV:
/en/page/subpage/
/sv/sida/delsida/
/no/page/under/

NO:
/en/page/subpage/
/sv/sida/under/
/no/page/under/

To fix that I changed the code on line 4 to test $langAspect->getId() does not match sys_language_uid to:

                $mountedPage = $pageRepository->getPage($mountedPage, false);
                /** @var \TYPO3\CMS\Core\Context\LanguageAspect $langAspect */
                $langAspect = $pageRepository->getLanguageAspect();
                if(($langAspect->getId() !== $mountedPage["sys_language_uid"] || !$pageRepository->isPageSuitableForLanguage($mountedPage, $langAspect)) &&
                    $mountedPage["sys_language_uid"] > 0 &&
                    $mountedPage["l10n_parent"] > 0
                ) {
                    $mountedPage = $pageRepository->getPage($mountedPage["l10n_parent"]);
                }
                $mountRoot = $pageRepository->getPage($mountRoot, false);

The problem also occurs with TYPO3 v10.

In my opinion this problem is already caused when creating the MP parameter.
I think that the "FirstPageUid" parameter should always point to the original Page-Uid and not to the Page-uid of the translation.
If I replaced the following line in TYPO3v9 at the class "PageRepository->getMountPointInfo" (line 1355)

--- PageRepository.php.orig    2020-06-30 07:29:06.140000000 +0000
+++ PageRepository.php    2020-06-30 07:29:37.110000000 +0000
@@ -1352,7 +1352,7 @@
         }
         // Set first Page uid:
         if (!$firstPageUid) {
-            $firstPageUid = $pageRec['uid'];
+            $firstPageUid = $pageRec['l10n_parent'] ? $pageRec['l10n_parent'] : $pageRec['uid'];
         }
         // Look for mount pid value plus other required circumstances:
         $mount_pid = (int)$pageRec['mount_pid'];

then the translation of the slugs works correctly for me.

Patch set 2 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/64952

Patch set 3 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/64952

Patch set 4 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/64952

Patch set 4 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/64952

Patch set 5 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/64952

Patch set 6 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/64952

Patch set 7 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/64952

Patch set 8 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/64952

Patch set 9 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/64952

Patch set 10 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/64952

Patch set 11 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/64952

Patch set 12 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/64952

Patch set 1 for branch 10.4 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/66756

Patch set 2 for branch 10.4 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/66756