Would there be any negative effects from uploading German content (job advertisement) to a page template that's tagged up as being in English?
We will not be able to edit the meta tags to signal that this page isn't in English.
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There should not be any negative effects if the majority of the content is in English.
According to Google:
Google uses only the visible content of your page to determine its language. We don’t use any code-level language information such as lang attributes. You can help Google determine the language correctly by using a single language for content and navigation on each page, and by avoiding side-by-side translations.
As a result, I would make sure that all content other than the advertisement is in English.