For the first time in my life, I've found a small explanation of mother tongue in Punjabi. I found it on this website. Below I'm posting the text transliterated into Roman Characters so that those unfamiliar with the script can read it:
Paidāish Yānī Jamāndrū Bolī
Jamāndrū zubān hī oh pehelī zubān hundī he jehḍī bagair kisī dūsrī zubān toṃ asar kabūl karan toṃ bace nūṃ ā jāndī he, te eho hī oh zubān hundī he jehede vic ām taur te soceā jāndā he. Beher hāl eh hakīkat he kī jeheḍī zubān vic vi sahī bolaṇ dā kasad hove ose hī zubān vic socṇā vī cāhīdā he. Kheālāt dī zubān sirf oho hī ik hundī he jeheḍī ohdī jamāndrū yānī peheli zubān hundī he cāhe oh angrez bacca hī kiyoṃ nā hove jeheḍā pehelī zubān angrezī sikh ke panj ceṃ sāl dī umar de magroṃ lagātār panjābī bolaṇ lag jāve (te fer kisī dūjī zubān dī havā ohnū nā lage) tāṃ panjābī hī usdī jamāndrū yā pehelī zubān kahāvegī te panjābī vic hī oh socegā.
Here's a translation of the above paragraph:
Mother tongue is the first language that a child learns prior to being influenced by other languages and it's the same language in which a person usually thinks. Meanwhile, a reality is: one ought to think in the language in which one can express himself well. More than often one only thinks in the language that is his or her mother tongue; this is true even in case of an English child who learns English till the age of five or six and then starts speaking Punjabi. In that case, if the child doesn't have a contact with any language other than Punjabi, Punjabi will become his mother tongue.
I know this translation isn't perfect and it's because I don't know what asar and kasad mean and I find it hard to translate Paidāish Yānī Jamāndrū Bolī into English.