Actually the extension doesn't have problem running in Spanish. The problem is that the month (example: September) is written with the first letter in uppercase. How can I have the month with everything in lowercase. In Spanish we don use the month in uppercase. Thanks!
If you use this, and still you have capitals in months names, I am afraid this is related to your Spanish Joomla translation, not to n3t Coming Soon plugin. You can override this directly in Joomla Administration. Just go to Extensions - Language Manager - Overrides and click "New" Button.
On the right side search for the name of the month in your language, it should find two constants, one with all months and second only for one month. Select the one with only one month name and correct the translation in Text field on the left side. Do this for all 12 months.