Like Chadori said, a spritefont only uses characters you've assigned to it. If you want full support for all characters you're better off using webfont that supports these characters.
The same would happen in any game engine or game for that matter. When old games were localized the devs removed Japanese characters to make way for English ones.
The game I have on Steam supports tons of languages, so I had to add each character used in these languages to the spritefont. In my case I even support the Chinese language. To do that I asked the translator to send me every character used along with the translated dialogue, so instead of adding all possible characters to the spritefont, I only used those that I knew were necessary - still it was a LOT.
imgur.com/a/LuzMxSS
But for name input, there's no way out I think. Your best bet would either using webfonts, or making a keyboard in your game that showed the available letters that the player can use.