Plurals, inflection and word genders
tl;dr: Be prepared: Some languages have more than one plural form and some languages have genders for words.
You might have learned that there is a singular form and a plural form. That is usually the case in English and is true for some other common languages as well. But it is not true of all languages. For example, Chinese has no plural forms of a word itself; instead, the plural is indicated by measurement words and counts. In contrast, Russian has multiple plural forms depending on which number the count ends in: 1 ruble is 1 рубль and 2 rubles is 2 рубля, but 51 rubles is 51 рубль and 100 rubles is 100 рублей. Unfortunately, some localization systems do not take into account the existence of multiple plural forms, so the translator has to work around the issue by including all the possible forms in one translation: 100 рубля/ей. If possible, favor a localization system or format that supports multiple plural forms — especially if you are targeting markets speaking Russian or Arabic.
Another big difference is the inflection of words in some languages. For example, in English, the word "office" does not change in the following examples: to the office and from the office. But in Finnish, there are no prepositions. Instead, the word inflects: toimistolle and toimistolta. It could be tempting to cut corners in localization and join strings like this: 'send to ' + i10n($target), but if the target language inflects words, the result will be impossible to localize correctly. A similar issue is caused by word genders: a word such as "cat" or "dog" is either a/the cat or a/the dog in English, but in German, a cat is eine Katze and the cat is die Katze, while a dog is ein Hund and the dog is der Hund, because the article changes along with the word's gender.