Depending on the data you want to load, you might need to pass 'root' or something different as $locale.
Here is a list of examples and the corresponding link to the file that gets loaded:
ResourceBundle::create('de', 'ICUDATA-brkitr', false)
https://github.com/unicode-org/icu/blob/release-65-1/icu4c/source/data/brkitr/de.txt
ResourceBundle::create('root', 'ICUDATA-curr', false)
https://github.com/unicode-org/icu/blob/release-65-1/icu4c/source/data/curr/root.txt
ResourceBundle::create('root', 'ICUDATA-translit', false)
https://github.com/unicode-org/icu/blob/release-65-1/icu4c/source/data/translit/root.txt
ResourceBundle::create('likelySubtags', 'ICUDATA', false)
https://github.com/unicode-org/icu/blob/release-65-1/icu4c/source/data/misc/likelySubtags.txt