Anyone can fork it and quickly add the i18n (or just translate into a different language) for their own purposes. People will likely want to contribute i18n. People may fix or improve things for you.
Of course, it's entirely up to you - but I've appreciated half-complete software countless times before.
That may be true.
> [...] you can't say they violated the GPL.
That does not necessarily follow.
If they used GPL-licensed code in their product, they may be obligated to provide the source code to that product's consumer.