Uncaught RangeError: Invalid string length when appending to a string in JavaScript

Your 2-D array accesses are incorrect, but the main problem is that you're re-using the variable i in an inner loop:

for (i = 0; i < strLength; i++){
    page = page.concat(spanishTerms[i] + '<br>');   
}

That i will be the same i as in your outer loop. Thus, the error you're getting is telling you that you're building up a massive string that exceeds the capacity of the runtime system. Declare a new variable for that loop.

Accessing a value from an array of arrays requires two sets of [ ]:

entry[j][0]