Looking at the screenshot you posted, it looks like you have a Spanish locale file. It could be just that the keys for those buttons are missing.
They are taken from the locale file. See the localization example:
Screenshot 2021-12-01 at 11.00.26.png (44.7 KiB) Viewed 699 times
Your locale file needs this section for those buttons (Obviously with the text translated):
Calendar : {
Today : 'Today',
next : range => `Next ${range}`, //<---- these are the buttons
previous : range => `Previous ${range}`,
plusMore : value => `+${value} more`
}
Ok my problem was the version of the Calendar component, I have the v4.2.6, and with that version the Keys next and previous do not translate with the Custom Locale I made in Spanish, but after I upgrade to v4.3.3, the problem was solved.