Wordpress - How can I uninstall a language?

I hope you have FTP access or some alike, so you can follow path as shown below.

Use any FTP tool (FileZilla or some) and enter your website.

1) Go to the wp-content folder
There you will see a folder named languages.

2) Go into the languages folder
There you will find a folder called plugins and themes.

3) Enter the plugins folder
You will see files which probably will end with -es_ES.mo and -es_ES.po which is for Español and maybe also -es_CO.mo and -es_CO.po which is for Español de Colombia.
All files as named above in this folder you can delete without any problem.

4) Enter the themes folder (if you wish to do it also for the theme translations)
Follow step 3. (the deleting part)

Edit:
Ofcourse are there also translation files for WordPress core itself in the wp-content/languages folder, these -es_ES and -es_CO files you can also delete ofcourse.

Now you are done but remember they (or other) will be back when you switch the language again in the admin (Settings/General) and let WordPress update the translation(s) or doing it yourself by selecting to update all.

Hope this helped you out.

In order to reset to English only? You will have to remove all files in these folders:

/wp-content/languages
/wp-content/languages/plugins
/wp-content/languages/themes

Remember that your default language will be used. If you installed a wrong version the language might be still present. So you will have to overwrite your files with new installation zip or tar.gz file from Wordpress on English. If that is applicable.