Praise be to Allah.
To highlight the virtue of the Arabic language it is sufficient to note that it is the language of the Holy Qur’aan, the eternal miracle, which was revealed to be a way of life for mankind until the Hour begins. Allah, may He be glorified and exalted, chose the Arabic language to be the vessel which would carry this light and guidance to mankind. This is mentioned in approximately ten places in the Qur’aan, such as the verse in which Allah, may He be exalted, says (interpretation):
“And indeed We have put forth for men, in this Qur'ân every kind of similitude in order that they may remember.
An Arabic Qur'ân, without any crookedness (therein) in order that they may avoid all evil which Allâh has ordered them to avoid, fear Him and keep their duty to Him”
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:
The Arabic language is the symbol of Islam and its people.
End quote from Iqtida’ al-Siraat al-Mustaqeem, 1/519
Once the status of the language is clear, we will understand the reward and status of the one who teaches Arabic:
1.By teaching Arabic he is helping to spread this virtue among people, and teaching the language of the Holy Qur’aan and the language that was chosen by the Lord of the Worlds.
2.He is playing a role in the memorization and understanding of the Qur’aan, teaching it and acting upon it. He is also playing a role in spreading understanding of the Prophet’s hadeeth, preserving it, explaining it and acting upon it. Any branch of knowledge that is connected to the Qur’aan and Sunnah is praiseworthy knowledge for which the one who teaches it will be rewarded.
3.The one who teaches Arabic is the direct cause of the spread of Islamic knowledge and education of people, because understanding Islam is based on understanding of Arabic language. So whoever helps the people to understand it has helped them to understand Islam, and has become a means of spreading goodness among people.
4.Especially since the effects of learning a language are not limited to the person who learns it only; rather they also spread to the people around him. When the person who learns Arabic studies and reads Islamic sources, he becomes a means of transmitting that culture that he reads to the culture whose language he speaks. This is undoubtedly part of spreading Islam and calling people to it.
5.The reward that the one who learns Arabic attains because of his worship and the manners that he learns from Arabic, the one who teaches him will have a similar reward, in sha Allah, and that will not detract from the reward of the one who does those things in the slightest. The Prophet (blessings and peace of Allah be upon him) said: “The one who tells another about something good is like the one who does it.” Narrated by Muslim, 1983.
Whoever does not have an excuse for refusing to teach Arabic to the one who needs it has deprived himself of a great reward, and has fallen short in his duty towards his ummah and his religion, as he is expected to be a caller to it and the means of spreading it and calling people to it, and he is content to be with those who fail to convey this religion and who are too lazy to call others to it.
And Allah knows best.