tl;dr
It was fine for ordinary conversation and non-creative writing, but perhaps not good enough to capture the expressivity of his Bengali originals.
Deets
Tagore was curiously diffident about his English. On 6 May 1913, the year after the English Gitanjali had been published to great acclaim, he wrote a letter in Bengali to his niece Indiradevi Chaudhurani. Chaudhurani's own partial translation of this says:
That I cannot write English is such a patent fact that I never had even the vanity to feel ashamed of it. If anybody wrote an English note asking me to tea, I never felt equal to answering it. Perhaps you think that by now I have got over the delusion. By no means. That I have written in English seems to be the delusion. (p. 3)
Yet this same letter goes on to say that he felt impelled to translate his poems into English:
I did not undertake this task in a spirit of reckless bravado. I simply felt an urge to recapture through the medium of another language the feelings and sentiments which had created such a feast of joy within me in the days gone by. (p. 3)
He seems confident enough that the act of translating into English would enable him to "recapture" his "feelings and sentiments"; this comports uneasily with the claim that he feels incapable of responding to a polite social note in that language.
In the long introduction to his 2011 edition of Gitanjali, William Radice translates a subsequent portion of this letter:
In the English language there are all these slippery things like articles, prepositions, "shall" versus "will": they can't be got right with common sense—they have to be learned. I have the notion that they're all living somewhere in my "subliminal consciousness" like worms underground. When I let go of the rudder and sit down to write with my eyes shut, they all come creeping out of the dark to do their stuff—but if I look at them in the light of full consciousness they wriggle off again all higgledy-piggledy—so in the end I feel that I can't rely on them at all. That's why it's still true to say that I don't know English.
What Tagore is saying here is that he is self-conscious about his English. As long as he doesn't think about it, he manages to wield the language fluently enough; as soon as he begins to reason his way to a conscious understanding of the rules of the language, he loses his way.
However, his fluency in the language was never raised as an issue in any of the social interactions he had with English speakers in India, the UK, and the US. Radice's Appendix E contains a narrative by one John Rattray about his meeting Tagore in 1938. Rattray says Tagore "spoke in English with a fine choice of words". Likewise, Radice discusses the enthusiastic accounts Tagore's audience in London gave of the 1912 evening where he first read out his own translations at a soirée arranged by the painter William Rothenstein. None of the accounts feels the need to make any sort of allowance for Tagore's English; they all suggest that Tagore held his own perfectly well in that language.
Tagore's letters in English read just fine as well. For example, Radice quotes one to Thomas Sturge Moore. Tagore bemoans the poor job he has done with his own translations:
I am convinced that I myself in my translations have done a grave injustice to my own work. My English is like a frail boat—and to save it from an utter disaster I had to jettison the most important part of its cargo. But the cargo being a living one it has been mutilated: which is a literary crime that carries its own punishment.
As with the letter to his niece, here too Tagore's words can't be taken entirely at face value. There is a lack of concord between what is being claimed ("I don't know English") and the medium in which he expresses that claim (a perfectly well-written letter in that language).
Tagore's speeches on various occasions also show a better than adequate command of the English language. In 1917, during a tour of the US, he delivered a lecture on "Nationalism in India". The lecture was written specifically for the occasion, and therefore originally in English. Here is a well-known passage from the essay:
Many people in this country ask me what is happening as to the caste distinctions in India. But when this question is asked me, it is usually done with a superior air. And I feel tempted to put the same question to our American critics with a slight modification, ‘What have you done with the Red Indian and the Negro?’ For you have not got over your attitude of caste toward them. You have used violent methods to keep aloof from other races, but until you have solved the question here in America, you have no right to question India.
The passage is eloquently argued and does not suggest that the writer is struggling to express himself in the English language.
Finally, we have Tagore's translations of poetry from English into Bengali. In 1886, long before he began translating his poetry into English, Tagore published a collection called কড়ি ও কোমল / ka.Di o komal, "Sharps and Flats". The collection included a group of about 15 poems by poets such as Percy Shelley, Christina Rossetti, and Elizabeth Barrett Browning, among others, rendered into Bengali by Tagore. The poems are collectively called বিদেশী ফূলের গুচ্ছ / videshii phuuler guchchha, "A Bouquet of Foreign Flowers." At the age of 25, then, Tagore was comfortable enough to be reading English poetry and translating it, quite competently, into Bengali.
None of this negates the fact that Tagore's translations of his own works into English did them no justice. Tagore himself diagnosed the problem in a letter to Ezra Pound, which Radice quotes in a footnote to his introduction:
Then again I do not know the exact value of your English words. Some of them may have their souls worn out by constant use and some others may not have acquired their souls yet.
This is somewhat cryptic. I think it means that Tagore knew that English was always a second language to him. He did not have an instinctive sense for the exact word that would capture his meaning; he was in danger of lapsing into cliché ("souls worn out by constant use") or newfangled strangeness ("may not have acquired their souls yet"). Anybody who has studied a second language can sympathize. It is a lot easier to translate from the second language into the first rather than vice-versa. And while expository prose might be easy enough to translate, translating creative works requires deftness in the target language. Tagore's Bengali originals are revolutionary in their use of language—his diction and inventiveness transformed Bengali literature. He lacked the capacity for the same deft originality in English.
Exacerbating the issue was the slapdash way in which he went about his translations. He found the task of translation tedious; but demand for his work was high, and he was under pressure to deliver work to his publishers. In 1913 itself, only a few months after the publication of the English Gitanjali, he complained to Rothenstein:
Now it is a mere business and it tires me. This cold blooded literary craftsmanship, this weighing of words and expressions is utterly wearisome. I am pining for the touch of life ... (Quoted in Radice's introduction)
For Tagore, translation was mere craftsmanship, dull and lifeless. It seems fair to say that his command of English, while fluent enough for transactional interactions, was not good enough to spark a creative impulse. His letter to Indiradevi Chaudhurani mentions that he set about translating his poems into English as a way to recapture the joy he had felt and had been able to express in Bengali. As such, he was able to proceed pretty unselfconsciously with that experiment. But once that experiment succeeded, and once he was tasked with translating more of his work in short order, his self-consciousness kicked in. At that point, whether he found any joy in English itself, whether he had any desire to explore the possibilities of that language as a literary medium, is open to question. The unfortunate combination of haste and lack of interest probably accounts for the poor quality of his own translations.
References
- Tagore, Rabindranath. "Genesis of English Gitanjali". Letter to Indiradevi Chaudhurani, 6 May 1913. Trans. Indiradevi Chaudhurani. Indian Literature 2:1 (1958–1959): 3–4.
- ———. Gitanjali: Song Offerings. Trans. and intro. William Radice. New Delhi: Penguin, 2011.