One example is trying to write English with an Arabic alphabet. Arabic doesn't have a /p/ sound, so you have to use the closest approximate sound in Arabic, usually the /b/ sound. But then the English words pig and big get transliterated exactly the same way.
The German alphabet actually has two of the same letters as English, V and W. However, in German the V is pronounced like English F, and the German W is pronounced like English V. So if you take the car brand of Volkswagen, or VW, in English it's 'vee-double you', but German it's 'fow vay'.
Hebrew has a letter for a sound that doesn't exist directly in English, the letter
נ , which in English is usually written with the letters 'ch', as in Bach. It's a harsh sound in the back of your throat. So that's why you'll see the Jewish Holiday of
נוכה sometimes written as Chanukah, or Hanukkah, or other ways in English.
Many Semitic languages don't write out the vowels in a language. You just write the consonants, or the root word. If necessary you can denote the vowels with diacritical marks, but that's not usually done. So I'd write 'cat' as 'tc' in Arabic or Hebrew (because they're written right to left), and you'd have to figure out from the context of the sentence if I meant cat vs. cot. vs. cut vs. cute vs. cote.
Xhosa, an African language uses
tongue clicks, which are extremely rare in languages. This is a really good example, because of not only the tongue clicks, but the fact that Xhosa doesn't have its own writing system. How do you write your language in a language that doesn't have a writing system?
Then you have the problem that not all languages use alphabets as a writing system. Some languages are written with a syllabary, where you have one sound for every syllable in the language. So the English words 'son' and 'sun' would be written exactly the same way, because they have the same syllabic sound. Japanese is like this.
Other languages are logographic, meaning you have pictures for the words, and you combine the pictures to create words. You have a picture for the verb 'run', and a picture for the noun 'water'. Instead of a new word for 'river' you combine the pictures for 'run' and 'water', to meaning running water. Which could be a river, or maybe just water from the tap. Chinese and Ancient Egyptian hieroglyphics are examples of this.