In an interesting book I read recently, What Language Is by John McWhorter, the author discusses why some languages appear a lot more complicated or ‘ingrown’ than others. He gives the example of Persian and Pashto, two Iranian languages spoken in a number of countries in western and central Asia. Whereas Persian has more or less regular and simple verb conjugations, in Pashto the verb endings and other aspects of the language are much less regular. This is because Persian was the language of a large empire in which many people learned Persian as adults, and few did so perfectly, so many of the irregularities and other complex aspects of Old Persian were regularised and simplified. This process didn’t happen with Pashto, so the language is still ingrown.
Other languages that are or have been used as colonial languages or lingua francas with many adults learning them imperfectly have undergone a similar process of simplification. These include English, Mandarin Chinese, colloquial spoken varieties of Arabic, Indonesian and Swahili. According to McWhorter, these languages could be considered abnormal as many of their irregularities and eccentricities have been levelled out. As a result they are relatively easy to learn, or at least somewhat less difficult than more ingrown languages.
One example a particularly ingrown language is Navajo, which even linguists find superlatively forbidding. Some even claim that it’s not possible to learn it after childhood. Apparently none of the Navajo verbs follow a regular pattern, and regularity is notably absent in other parts of the language.
So if you’re struggling to get to grips with Spanish or Mandarin, it might be of comfort to you to remember that you’re not learning Navajo or a similarly ingrown language.