Created attachment 95226 [details]
ODS file

This is using german spell check by default, I don't know if this is different with another default language:

1. Open Example2.ods
2. As you can see a lot of the English text in Cell A1 is marked with a red underline. That is because the language of this Cell is German.
3. Highlight A1 and press Strg + 1 to go into the „format cell“ menu.
4. Go to „font“ and change the language to „English (UK)“
5. As you can see the English text still has a red underline despite having the correct language now. This is incorrect.
6. Now Highlight A1 and press F2 to edit the Cell.
7. As you can see now the German text get's a red underline. This is correct.
8. Press Esc to leave edit mode. Now the English text has a red underline again. This is incorrect.
9. This bug persists even if you Save & Close the file.
10. Now make a right click on A1 to enter edit mode.
11. As you can see now the red underline is & stays correct.

In short:

If you change a cell's language it doesn't show.