As I observe from some reading that tulsidas was a awadhi poet. But as I see ramcharitmanas is said to be a translation of Ramayan from valmiki, my question is how could tulsidas translate it if he couldn't understand Sanskrit. (One has to be a proficient in sanskrit to be really able to translate a epic )
Who said Tulsidas didn't know Sanskrit? Did you read the opening invocations (Mangalacharan) before every Kanda?
For your benefit let me quote only the Mangalacharan of Balkand.
वर्णानामर्थसंघानां रसानां छन्दसामपि।
मङ्गलानां च कर्त्तारौ वन्दे वाणीविनायकौ ।।1।।
I worship Saraswati (the goddess of speech) and Lord Ganesh, the originators of words represented by the alphabet, their meanings, rasas, meters, and the givers of all blessings.
भवानीशङ्करौ वन्दे श्रद्धाविश्वासरूपिणौ।
याभ्यां विना न पश्यन्ति सिद्धाःस्वान्तःस्थमीश्वरम् ।।2।।
I worship Goddess Parvati and Her consort, Shankara, the embodiments of reverence and faith, without then even the adept cannot perceive God present in their own heart.
वन्दे बोधमयं नित्यं गुरुं शङ्कररूपिणम्।
यमाश्रितो हि वक्रोऽपि चन्द्रः सर्वत्र वन्द्यते ।।3।।
I worship Lord Shankar, the eternal preceptor, who is all wisdom, and resting on whose forehead, the crescent moon, though crooked in shape, is universally worshipped.
वन्दे विशुद्धविज्ञानौ कबीश्वरकपीश्वरौ ।।4।।
I worship the Lord of the poets, Valmiki and the Lord of monkeys, Hanuman who are full of pure knowledge and enjoy freely in the holy woods of the glories of Sita and Ram.
सर्वश्रेयस्करीं सीतां नतोऽहं रामवल्लभाम् ।।5।।
I bow to Sita the beloved consort of Ram, who is responsible for the creation, sustenance and the dissolution (of the universe), removes afflictions and showers all the blessings.
यन्मायावशवर्तिं विश्वमखिलं ब्रह्मादिदेवासुरा
यत्सत्वादमृषैव भाति सकलं रज्जौ यथाहेर्भ्रमः।
यत्पादप्लवमेकमेव हि भवाम्भोधेस्तितीर्षावतां
वन्देऽहं तमशेषकारणपरं रामाख्यमीशं हरिम् ।।6।।
I worship Lord Hari, known by the name of Ram, who is superior to and lies beyond all causes, whose Maya (illusive power) holds sway over the entire universe including the gods Brahma (the Creator) etc. and the demons, whose presence lends positive reality to this world of appearances like a rope appears to be a snake and whose feet are the only ship for those eager to cross the ocean of mundane existence.
नानापुराणनिगमागमसम्मतं यद् रामायणे निगदितं क्वचिदन्यतोऽपि।
स्वान्तःसुखाय तुलसी रघुनाथगाथा-भाषानिबन्धमतिमञ्जुलमातनोति ।।7।।
For the gratification of his own self Tulasidas brings forth this very elegant composition relating in common parlance the story of the Lord of Raghus, in accordance with the various Puranas, Vedas and the agamas (Tantras), and incorporates what has been recorded in the Ramayana (of Valmiki) and culled from some other sources.
If you're interested you can look up the Mangalacharans of the other 6 Kandas.
Anyway, Tulsidas was a disciple of Narahari Das, from the Ramanandi Sampraday. He didn't want Ram Katha to be in Sanskrit not because of his lack of knowledge, but for the common public's. He was very fluent in Sanskrit and Awadhi both. He was thereby titled as Mahakavi, or great poet, and was rightly so.
Again Ramcharitmanas is not a translation. A translation would mean the same verses of Ramayana, now available in Awadhi or any other language. Tulsidas knew the whole story, and had written it in his own style. Plus he himself mentions that he had taken other references from Puranas and Nigam-Agamas in this Mangalacharan itself. So again, no, Ramcharitmanas is not a translation of Valmiki Ramayana, but an original Awadhi work which had the same subject as Ramayana -> Ram's life and deeds.