[SalesForce] How to use Custom Labels inside of an expression on a custom button

Sdry, I have wanted to do exactly this ever since Custom Settings came out. The problem, though, is that there is no Apex API for Custom Labels (click the link to vote for the idea for it!), so Custom Labels cannot be dynamically retrieved or requested within Apex. However, it CAN, as of Spring 12, be done in Visualforce, using Dynamic Components.

Here, I think is what you are looking for in Apex (that cannot yet be done):

Custom Settings + Custom Labels IN APEX (**not yet possible**)

// Get the stored Custom Setting record for User Preferences
User_Prefs__c p = User_Prefs__c.getInstance(UserInfo.getUserId());

// Get the stored ISO Code
String isoCode = p.ISO_Code__c;

// Get the running user's language
String languageKey = [
    select LanguageLocaleKey from User 
     where Id = :UserInfo.getUserId() 
     limit 1
].LanguageLocaleKey;

// Get the translation of the ISO Code corresponding to this user's language
// **THIS IS NOT POSSIBLE YET IN APEX**
String translatedCode = System.Label.getLabel('ISOCode_'+isoCode).getTranslation(languageKey);

And here would be an equivalent if you are using Visualforce:

Custom Settings + Custom Labels IN VISUALFORCE (possible right now)

/* APEX CONTROLLER */
public class Controller {

    public String getStoredISOCode() {
        // Get the stored Custom Setting record for User Preferences,
        // and return the ISO code
        return User_Prefs__c.getInstance(UserInfo.getUserId()).ISO_Code__c;
    }

}

/* VF PAGE */
<apex:page controller="MyController">

    <b>ISO Code</b>{!storedISOCode}
    <b>Translated</b>{!$Label['ISOCode_' & storedISOCode]}

</apex:page>

Even though it depends on teams to teams when it comes around best practices, but as for your question

Any thoughts on what makes for good naming conventions for a pile of Custom Labels?

I have usually taken an approach of reflecting the intent of the custom label in its name. Have also tried to keep the naming convention seamless so that it can be organized. I have mostly used below approach from best practices around naming conventions of custom labels:

• Use uppercase letters
• Separate words within the name using an underscore
• Reflect the intent/characteristic of the label
• Keep the overall length of the label name short

As an example, let's say if I have a some sort of legal text to be displayed somewhere, I would create a custom label as say, LEGAL_DISCLOSURE_TEXT. Similarly if I had something related to say finance, I would have it as FINANCE_COLLECTION_DAYS.

Again, it all depends on which approach you take, but usually starting right from design helps it from future maintainability and scale-ability perspective.