In what kind of contexts should you use 处于/处在?
The different between 「处于」and 「处在」is answered well in @dan, and I prefer to explain why Pleco use "be".
Actually the mean of the two word can be explained to "be in", and it's used to in a condition or situation.That means it's usually used as 「处于/处在...... 中」，but it is not in this form absolutely. We can also use like this:


  「我们必须和一切爱好和平自由的国家和人民团结在一起……使我们的保障人民革命胜利成果和反对内外敌人复辟阴谋的斗争不致处于孤立地位。」 ————毛泽东 《中国人民站起来了》


And if you want more further explanation，you can visit http://www.hydcd.com/zidian/hz/10444.htm