Geneanet and FamilySearch have the option of choosing one of 10 different languages. Geneanet has English and 9 European languages. Family Search has as four languages in non-Latin alphabets in its options, Russia, and three Asian languages.
I do not think Geneanet translates user entries into the chosen language, but just keeps them as entered (I have used pages where user entry is in Spanish and English — probably cut and paste from other sources), but all the webpage buttons, labels, and so on are in that language.
You change from one language to another by selecting the language from a drop down menu. You can make the selection whenever you want.
It is surprising that Spanish is not in either one, especially considering the LDS efforts in Latin America.
So, it certainly could be done.
Edit ...
I was just experimenting with FamilySearch/Family Tree. I selected German as the language, and had it generate a family group sheet with sources. All the form labels were in German, but the user entered text was as entered by the patron.