But all of the sounds of a language are generally acquired before puberty by a native speaker. Surprisingly, though, the hardest sounds to learn may be those that are similar to, but just a bit different from, sounds in your native language. It seems to be very difficult to overcome the tendency to keep using the familiar sounds from your native language. In this sense, your native language causes 'interference' in your efforts to pick up the new language. Again, it depends on whether we're talking about a first or second language.
Children acquire their native language effortlessly, regardless of the language.
Learning another language later on, however, is a different matter. Some languages do have far more complicated word-building rules than others, and others have far more complex sound patterns or sentence structures. But despite differences in individual areas of a language, researchers have not found any one language or group of languages to be clearly more difficult or complicated in all areas. Learning a language that is closely related to your native language can be easier than learning one that is very different. French, Italian, Spanish, and Portuguese are all descended from Latin, so they are closely related, and a speaker of one can learn any of the others fairly easily.