If I recall correctly Twig doesn't support || and && operators, but requires or and and to be used respectively. I'd also use parentheses to denote the two statements more clearly although this isn't technically a requirement.
{%if ( fields | length > 0 ) or ( trans_fields | length > 0 ) %}
Expressions
Expressions can be used in {% blocks %} and ${ expressions }.
Operator Description
== Does the left expression equal the right expression?
+ Convert both arguments into a number and add them.
- Convert both arguments into a number and substract them.
* Convert both arguments into a number and multiply them.
/ Convert both arguments into a number and divide them.
% Convert both arguments into a number and calculate the rest of the integer division.
~ Convert both arguments into a string and concatenate them.
or True if the left or the right expression is true.
and True if the left and the right expression is true.
not Negate the expression.
For more complex operations, it may be best to wrap individual expressions in parentheses to avoid confusion:
{% if (foo and bar) or (fizz and (foo + bar == 3)) %}