Meaning of 'le' at end of sentence

Why is there 'le' in Wǒ xuéxí Hànyǔ yǐjīng yīgeyuè le ?  (I think it's module 1.2 or 1.3.)

The 'le' indicates that the action is completed.  So, he is saying "I studied Chinese for just a month".

But the English translation is "I have been studying Chinese for one month", which means that he is still studying it.  (He's not much of a model student if he only stuck with it for one month!) Also, Lin's question "Nǐ xuéxí Hànyǔ duōcháng shíjiān le?" has an English translation that likewise assumes that Dave's Chinese studies are not over. 

I know there are lots of subtleties involving 了 (le).  In this lesson, as I recall, they claimed that 了 is used to indicate past tense.  I think it is a bit inaccurate to think that way.  Other things that 了 can indicate is a change in state, like "it was cold yesterday, now it's hot" - 昨天很冷，今天热了 (zuótiān hěn lěng, jīntiān rè le).  There is probably something very subtle being conveyed.

I think it might be better to translate "Nǐ xuéxí Hànyǔ duōcháng shíjiān le?" to "how long have you studied Chinese?" and Dave's answer " Wǒ xuéxí Hànyǔ yǐjīng yīgeyuè le" should translate to "I have studied Chinese for just a month."  In this case, both question and answer are about the past, even in English, and neither precludes the possibility that Dave is not continuing to study.