The English interface is reasonably well-tested (many of my Chinese friends use it to practice their English), but other languages might cause overflowing UI elements because the translated text is longer than accounted for.
That problem also affects the English version in some places e.g. when adding a contact, the short introduction you can write to explain who you are and why you're adding them has a length limit. It's probably enough for Chinese text, but requires being very concise when using English.
Many features are designed to prevent virality. Comments on a friend's timeline ("moments") are only visible to your shared friends. Small chat groups can be bootstrapped by scanning a QR code or by typing a shared passcode simultaneously, but beyond 100 users newcomers need to be invited by a member first. Comments on public articles are hand-picked by the author and only the author can reply directly. There used to be a feature displaying trending articles, but it seems to have been removed.
Those limitations prevent regime-critical opinions from spreading too quickly, but they also mean that only users with a large social circle on WeChat are going to stick with the app.
The article mentions women being harassed via the "people nearby" feature. My impression is that it is mostly used by men looking for hookups and by "women" who are prostitutes or pretend to be prostitutes to scam those men.
Successfully entering a market without already-strong ties to China would have required making large changes about the way their app works.