English — is the main language on Upwork(oDesk). For most professionals in IT this is not a problem at all, but even if not, it should not delay the start of work.
About 98% of all work on Upwork(oDesk) is conducted in English, of course there are customers who want to communicate in Russian or Chinese, but it is rather the exception. So should be ready that all work will be conducted in English.
The Main way to communicate — emails, less - skype chat. Be prepared that you will have to conduct live chat with a customer, even if not using voice. It is not necessary to have good speaking English, but the ability to communicate fluently in English gives you opportunity to take an expensive project and lead a more productive work.
If for you it's hard to write in English, it should not stop you to apply for a job on Upwork(oDesk), because you can learn English during you tasks.
The spelling of grammar is not particularly important, the main thing that the customer and you can understand each other. Clients understand about the fact that English is not the main for you, the main thing - you can do what the customers want.
I tried to learn English from a school, but not luck. Even after university I can only read and only with a translator.
That time I had a passion to play computer games. This helps me to improve my skills, now I can read more words, but cannot write and talk.
When I got first job then most tasks required me to go to Google and make a search.
I read much on forums and in the Internet, thanks Google translate.
I started to work on Upwork(oDesk) with the following level: fluent reading technical articles, and write with great difficulty, no talk at all.
In a couple of months, I have learned to how effectively chat in Skype without Google translate.
And although my grammar still leaves much to be desired, for the customers it does brings any discomfort.
Working with clients I thought about an easy way to improve my English speaking. I did some research and found these web sites: italki, verbling and livemocha. I tried all of them, because they offer an option to find a free partner to exchange languages. Finally I found a great professional teacher on italki who developed my confidence and prononciation.
You can use this link to register on italki and get 100 points (their local currency).