Add Spanish translation and make `StringUtil` capitalize the first letter, not character

[altrisi]

This pull request adds a Spanish translation, and adapts StringUtil.capitalize slightly to be able to support its leading punctuation (¡).

StringUtil will now search for the first letter or number instead of choosing always the first character, meaning that strings leaded by punctuation (such as ¡ in my case) will still be capitalized. Number is also checked because I believe 5 mods is the correct spelling, not 5 Mods.

I was not able to get JiJ-related strings to trigger.

Also, I think the title "Incompatible mod set!" (and probably the button texts) should be translatable.