You can find symbols in two places in the Letter Stencil Maker.
The first being the + beside the textbox. This is where you can find the symbols for that specific font you're writing with. Please note not all fonts have symbols.
The second place is the symbols button in the Font tab. This is where you can scroll through all the symbols.
You can copy & paste foreign letters from a website into the Letter Stencil Maker . On Mac and iOS, you can enter foreign letters by pressing and holding on a letter. On Windows, you can use the builtin Character Map program.
This assumes the font has letters in that language. The Tinos font covers many alphabets. You could also switch your keyboard to a foreign language.
If you'd like us to add a font with support for a particular language, you can search here. We can only add fonts that are OFL/Apache licensed, so make sure to filter by or check that before requesting a font.