There are two elements to poor language as a career killer. The first one is pretty easy to address while the second presents a few more challenges.
The first one is profanity. There is but one rule regarding profanity in the work environment and that is don't use it. Of course when you are with your friends or talking to someone you know well one on one, well it might be okay to let the occasional swear word out into the conversation but otherwise, it really diminishes you in the eyes of almost everyone, even those who use a lot of profanity themselves. Sure this is unfair but this is also life.
I have seen employees get into an elevator and start talking to their friends about F*** this and so on, all the while the CEO of the company standing at the back getting a real sense of who the players are. Oops!
Swearing is a part of life. It just happens to be not particularly classy or well received by people in authority. Don't do it and you won't kill your career.
As a side bar here, I need to make a similar warning about disparaging remarks about stereotypes. Please be careful, no matter what your personal belief system might be, not to denigrate any faction of our global society. Nothing will kill a career faster than the perception that someone is racist or a bigot or intolerant. It does not matter if your comment is just a "funny" off handed remark. I the world of business, it is not funny, nor is it appropriate and the last thing you want to be labeled is intolerant. Remember that key question your boss has on his mind at all times: "Do I feel comfortable putting this guy in front of a major client?". If that client happens to be from another country, embrace a different religion or have a different sexual orientation and you have been heard cracking jokes at the expense of such individuals, the answer will be "no" and your career will be stalled.
The second element that I want to raise and which is a lot harder to manage, is that of using proper English (or any other language as may be appropriate) in conducting your business transactions. I am only familiar with English so I'll use this as an example, but I am sure similar items exist in other languages.
Using proper English is important not because you want to sound like a scholar, but rather because not using proper English will cause you to fail that important question regarding interacting with key clients. Obviously if English is not your first language, then some accommodation will be made automatically. However if English is your first language and you miss use terms, words and phrases, then your reputation and credibility will suffer.
For example, I regularly hear people use the term: "irregardless" which is actually not a word. Most of the time, people want to say "regardless" and yet somehow "irregardless" seems to step in.
Another example: People often incorrectly say: "I seen..." instead of "I have seen..." It is not my intention to try to provide all of the examples here since this page would be a mile long. My point here is that, proper use of language is key to making sure you are still in contention for future promotion.
The best thing you can do about this is to ask people who are articulate to provide you with feedback. Are you clear in your communication? Do you use words appropriately etc. This can be a humbling experience but consider the consequences of not raising your language skills. It could kill your career!