Thank you for your post.
Once you put a value into a field, FileMaker cannot perform any translation on the field except through a calculation. You may want to consider two different fields; one for Spanish and one for English. Each person would enter the information into the appropriate layout, and then you would have a script trigger that would place the translation in the other fields. For example, on your Spanish layout, the script trigger would evaluate "Si" and "No" (and empty) place either "Yes" or "No" (or empty) in the English field. When the English person then views the layout, the most up-to-date information will be visible in the correct language.
I know you just used one field, and I assume there may be many other fields that need translation. However, this may give you a good start.
Please let me know if you need clarification for any of the above steps.
For simple word/phrase substitution, you may be able to build a table of terms where equivalent terms have some value in common, but are labeled for respective languages.
Your table might look like this:
Key : Term : Language
1 : Yes : English
2 : No : English
1 : Si : Espanol
2 : No : Espanol
Now you can set up calculations and other data references that can match terms by key and selected language.