If you do not feel comfortable doing this, simply submit a ticket in the Client Centre over at IPS for assistance. I'm providing the basic steps here to assist anyone who wishes to attempt it themselves.
Depending on how a language pack author chooses to export a language pack, it can overwrite the default English (US) pack. This usually happens if you are provided with separate .xmls (such as members_language_pack.xml.gz) instead of a complete language.xml.gz file. It is preferable to use the latter 'complete' package where possible to avoid this.
If your default English (US) has been overwritten and you wish to restore it:
Taking a database backup is recommended before starting.
Visit the SQL Toolbox:
ACP > 'Support' tab > SQL Management > SQL Toolbox
Run the following query:
[sql]UPDATE core_sys_lang_words SET word_custom = '' WHERE lang_id = '1';[/sql]
This assumes that the default language pack is still ID1, and has not been deleted. You may need to adjust the query slightly, adding your prefix if your board started on the 1.x or 2.x series.
Assuming all went well, visit the Language Manager and choose to rebuild from xml. ACP > 'Look / Feel' tab > Manage Languages > and click the grey button on the right on the English (US) pack and choose 'Rebuild From XML'
Wait for this to finish. It may take a minute or two.
After this has completed, click the grey button again and choose 'Recache All...'
View your board again. The default should of been restored. If it has not, you may wish to check the default xml's have not been replaced (although this is unlikely) , the locations are as follows:
The official applications are stored in the same fashion, but are located in: \admin\applications_addon\ips\ (application name) \xml\
Once these have been replaced with defaults from the .zip's as appropriate, follow the restoration steps again.
If this fails, you will need to contact IPS for assistance with the restoration, although in the large majority of cases, this does work.