Probably no language can "fit" on a keyboard, if you mean "get the letter by pressing only one key". if i only use one key at a time, i can type a lot of english but i don't have any key for capital i etc, i can only use single quote, and so on. I can double that using the Shift key, one of the so-called Modifier keys. Then, in principle, I can further increase the number of letters by pressing alt then a key, likewise with ctrl, and ctrl-alt (though interactions with other software, such as Word, can complicate things). "Keyboard" is a bit of an abstraction, involving a physical object and various layers of software. With 104 keys and the right software, you can write 52 upper and lower case letters, the digits, some punctuation, and have a few spare keys for computer functions, which accomodates English (it would be very non-mnemonic). If you include sequences of key-presses composed of modifier key(s) + one regular key with only shift, control, alt as modifiers, and nothing else, you have 707 possible letters. Chinese is clearly out of reach, since there are more than 707 characters.
There are about 296 letters in Tigrinya script, although they are kind of decomposable to 40 core consonant shapes and vocalic modifications. Devanagari has a huge collection of letters, since apart from letters for "c", "t" etc there are also letters that conjoin letters such as क्ष which combines क and ष, and there are 1296 two-consonant conjuncts plus many 3 and 4 letter conjunctions like च्छ् and क्ष्ण्य. These are usually (?) dealt with automatically at the software level so that non-alphabetic languages with many letter shape variants (Tigrinya, Hindi, Arabic) don't require massive special keyboards. Given the right software, the hundred thousand characters of Chinese can be handled with qwerty keyboard.
In other words, it depends on what you mean by "fit". sequences of keystrokes enable an unbounded set of letters to be produced.