When dealing with mathematical symbols, most languages are pretty consistent, even those using alternatives to western Arabic numerals, and have been for a few hundred years.
The biggest thing you need to worry about is separators, which may vary considerably in shape and placement, and mirroring certain symbols (radicals, for instance) when the language is right-to-left.
Since + and - are not separators, the first issue does not apply. Since they are symmetrical, the second issue does not apply.
In short: + and - for increment and decrement should be readily understood in most languages. Whether they are the best symbol for a given app or audience is a more nuanced question, but there shouldn't be any concerns around ambiguity.