I have come across many who always thought themselves to have superb English proficiency. Among some of their traits:
1. All they know is broken English, and they don’t know much about grammar and spelling;
2. Given their broken English, they know few vocabularies. When they want to express something but become wordless, they borrow those terms from another language;
3. Whenever they need to write an email, work report or some articles, they tend to use some words such as can (auxiliary verb), few (adjective / determiner) and will (auxiliary verb) excessively. They thought this is how to express their ideas in writing. For those who read and write well, these writings are definitely amusing;
4. They can only understand half of the English news in the place they live. Even if they understand eighty to ninety per cent of it, they often find themselves struggling to figure out what is said and written in those news reports and pieces produced in UK, US, Canada, Australia, and New Zealand.
Language proficiency is all about keep on improving yourself. You either make progress or start to regress. Those who are complacent often need help from us the English translators and editors to polish and improve their writing so that their pieces are presentable and comprehensible for all readers across the globe.