Why is my pasted text being marked as a different language?

I have an MS Word 2016 document. I can do CTRL+A to select all, and in the Review ribbon -> Language group, choose Language -> Set Proofing Language and "Mark selected text as" English. "Detect language automatically" is turned off. For those encountering this issue who might not know/remember how to check for this, including myself as OP years later, here's a screenshot, focusing on the second checkbox at the bottom:

Then, I can copy English text from any other program - it could be a browser on an English page (e.g. copying the instructions on this page) or Wordpad or even a browser's own chrometext when all its settings are English, and when I paste it in, it's marked as German.

I've never used this computer for German correspondence and never set interface languages to German. I've only opened about a half dozen distinct Word documents since installation, and none of them were in German. In Language Preferences, English is the default. Why is pasted text marked German? More importantly, how can I stop that from happening, other than CTRL+A and selecting the language from the Language tool on the Review tab after every paste?

I recognize that language is part of text formatting. It's kind of the opposite problem of having non-English text marked as English, which could be blamed on the Web marking most language as English by default.

Solution 1:

Somehow, the language got changed in the style that text was being pasted into. Solution:

• In the Home tab, locate the wide Styles section and click the tiny arrow in the bottom right.
• Drag the popup area over to the left so it it becomes the Styles pane.
• After observing that pasted text is formatted with e.g. Standard (might be Normal or something else for you), hover over the paragraph symbol to the right of Standard.
• A small downward-pointing triangle should appear. Click it.
• Click Modify. (shown in screenshot)
• In the lower left corner of the Modify Style dialog, click Format, then Language.
• Set the language.

The red lines underneath text marked as the wrong language may persist, but triggering the start of a spelling / grammar check with F7 should clear that.

If the issue persists, check the styles in the Normal template for new documents.