As far as I know there is no convertNumberToWords
type method in twig or php. However, you could create your own. This could be done in several ways: twig macro, custom twig filter, or plugin variable.
Here is an example of a simple twig macro that uses a lookup table:
{% macro numberToWords(integer) %}
{% digits = {
0: 'zero',
1: 'one',
2: 'two',
3: 'three',
4: 'four',
5: 'five',
6: 'six',
7: 'seven',
8: 'eight',
9: 'nine',
10: 'ten',
11: 'eleven',
12: 'twelve',
13: 'thirteen',
14: 'fourteen',
15: 'fifteen',
16: 'sixteen',
17: 'seventeen',
18: 'eighteen',
19: 'nineteen',
} %}
{{ digits[integer] }}
{% endmacro %}
{% import _self as macros %}
{% for i in 1..19 %}
{{ macros.numberToWords(loop.index) }}
{% endfor %}
Creating a custom twig filter or a plugin would of course allow you to use php, which might provide a more robust solution. Doing a quick search I see there are a few examples that people have written, like this one.
If you want to do this via a plugin, you might try using lindseydiloreto's excellent craft-businesslogic as a starting point. Then just paste the php methods (from the example linked above) into the variables file (along with any dependency methods).