Each language has a dominant frequency range, and it may be the reason of not hearing other languages.
The audible frequency of human is 20Hz to 20kHz, but the full spectrum is too much to be totally used for language cognition. Just like the radio frequency, each frequency range is assigned different functionality, so that the full potential of the spectrum can be exploited.
However, redundancy eventually appears as some of them are not required, so we gradually becomes less sensitive to them until we can hardly hear those specific frequency range. That is also true for languages, each language occupies a dominant frequency range, which language speakers will be tuned be more sensitive to it and less sensitive to others, eventually fitting the frequency range of the language.
The frequency ranges of major languages are found in the following table.
UK English and US English, although sharing a common language, differs in the frequency range. UK English tends to have a higher pitch, which starts from 2000Hz and peaks at 12000Hz, while US English starts at a lower frequency 1000Hz and peaks at 4000Hz, which is only ⅓ of UK English, explaining the phonological differences of the two languages.
It is common for us to have languages with low frequency range, including French, Chinese, German, Spanish and Japanese, and the range is narrower, which is on average 2000Hz. Among them there are the three fastest languages, which we may conclude that a lower frequency allows us to have a faster pronunciation.
Languages with a medium frequency range are US English and Italian. The range is slightly broader than those with a low frequency range.
The only language with high frequency range is UK English, which peaks at 12000Hz.
There are also languages with a broad frequency range, which are UK English with a range of 10000Hz, Dutch with 3875Hz, Russian with 7875Hz and Portuguese with 3750Hz.
So we got two quantities of language frequency range, pitch and bandwidth. We may use them to explain the barriers in hearing other languages.
The problem of pitch is about inactivation of a specific frequency range. Exposure to that helps us gain the required cognitive ability.
The problem of bandwidth falls between two extremes, the inabiity to hear a broad spectrum of frequency or differentiate a narrow spectrum of frequencies. Again, exposure helps.
Listening to other languages is not only about pronunciation. It is also about frequency range.