OpenXcom is designed with customizability in mind, so everything that users might wanna change is available in external files that anyone can edit.
While English is the most popular language, not everyone can understand it, so OpenXcom supports Translations that allow you to localize the game in any language you need.
If you're the kind that likes your soldiers to have a specific theme to them, or just want to add your own country, you can customize the Soldier Names used for the new hires.
Sprites are also moddable, robin on the forums has written a recipe how it can be done effectively: Modding sprites
Rulesets / Mods
Rulesets form the basis of mods, they contain the values and parameters for every single element in the game, and can be easily tweaked with a text editor. See Ruleset Reference Nightly (OpenXcom) for the list of current possibilities. As you can see over there, OXCE is made for greater mod capabilities. However, development is ongoing and ruleset language may change (does not happen often, but still).
OXCE supports scripts. This allows modders to do much more than change existing parameters of given existing entities in an uniform and fixed way, of course.