It's already a borrowed word from Spanish, and is commonly used in Filipino.
For example:
* "Ang huling araw ng buwan ay Ultimo." (The last day of the month is Ultimo.)
However, if you're looking for a more native Tagalog expression for "Ultimo," you could use:
* "Huling araw" (Last day)
* "Katapusan" (End)
These options might be less common, but they express the same meaning.