Whether it's safe or not depends on what you are trying to do. One potential problem, if you're trying to pick apart something like floating point numbers, is that decimal points aren't always used as the character for digit grouping. Commas are also seen, in which case the thousands separator is switched to a decimal point. (Of course, it isn't a "decimal point" anymore, but...)

Many toolkits provide an abstraction for this character to avoid ambiguity. I don't know if Qt does this, but I wouldn't be surprised. If so, you should use that pseudo-character for comparisons, as it will change depending on the user's localization.