Thank you for your report.

About your question: It might be a good idea to ask it on Slack in the #typo3-cms-coredev channel.

Also, if your issue is no longer reproducable, please let us know in the slack channel, so that this issue can be closed.

Thank you in advance!

Sybille

I have used the pre-render hook of PageRenderer to fix that:

    public function addLanguageFile(array $parameters, PageRenderer $pageRenderer)
    {
        $pageRenderer->addInlineLanguageLabelFile('EXT:lang/Resources/Private/Language/locallang_mod_web_list.xlf');
    }

Stefan

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/58186

Problem also exists if you click on the delete button. (9.5-dev)

But the patch fixes the problem with the modal for the page tree action.

Patch set 1 for branch TYPO3_8-7 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/58318

I tried to apply the patch with a TYPO3 version 8.7 but the problem remains the same.
Even if I clear all caches.

But I can confirm Stefan Froemken's workaround : pre-render hook of PageRenderer.

So I think a simple solution could be to add this code in \TYPO3\CMS\Backend\Controller\BackendController :

    protected function renderJavaScriptAndCss()
    {
        if (TYPO3_MODE === 'BE') {
            $this->addInlineLanguageLabelFile('EXT:lang/Resources/Private/Language/locallang_mod_web_list.xlf');
        }
    ...
    }