'my third language' is a correct and usable phrase in written English
For example, you can use it in a sentence like this: "I'm learning Spanish as my third language after English and French."
Hamid had forgotten Urdu, forcing him "to relearn my first language as my third language".
I went through the courses, I did all the training, I sat an English language test (you'll be surprised to learn that even though English is my third language, I passed) and the naturalization test.
In my defense, however, I did think for myself; I just wrote badly, due to the fact that English is my third language and I was still wearing donated jackets from Camp Pendleton, the Vietnamese refugees processing center in Southern California where my humble American life began.
In Off The Record, the actor and theatre interpreter Alex Jones, who was born deaf, mostly used sign language – "my first language" – but later recited in speech a personal poem displayed on the wall.
My first language is Xhosa.
That's my first language.
French was my first language.
I was always speaking my first language.
Yet English is my first language.
I learned English, which is my second language.
