The + operator always means numerical addition in JSP Expression Language. To do string concatenation you would have to use multiple adjacent EL expressions like ${str1}${str2}.
If I read your example correctly this could be written as:
${var1 == 0 ? 'hi' : 'hello '}${var1 == 0 ? '' : var2}
Edit
Another possibility would be to use JSTL, which is longer but might be clearer if there is more text that depends on var1:
hi
hello
The c:out might not be needed, depending on the JSP version.