Does "couldn't" mean "the subjunctive mood" in the following sentence?
I couldn't make myself understood in English.
As StoneyB and BillJ comment, there is nothing in the sentence to suggest the subjunctive mood. It is just a simple statement of ability.
However as TRomano mentions you can add more to the sentence to make it subjunctive, by turning it into a hypothetical case.
I couldn't make myself understood in English even if my life depended on it"