I've seen many ill-defined TLAN (LANGUAGE) tags lately, so here is a short explanation - and a proposal for "instrumental" titles.
From ID3.org: "TLAN: The 'Language(s)' frame should contain the languages of the text or lyrics spoken or sung in the audio. The language is represented with three characters according to ISO-639-2. If more than one language is used in the text their language codes should follow according to their usage."
All other use, like putting "German" or "English" in this field is ill-defined.
For sorting, the "bibliographic" (i.e., "ger" for German) and "terminology" (i.e., "deu" for German) language codes should be considered equal, since many people are unsure which to use, and many languages have both variants. I recommend using the "bibliographic" versions, i.e. prefer "ger" over "deu" for German.
Also, there are songs out there sung in more than one language, so these should be found/sorted according to whatever is in the tag (i.e., "ger" and "eng" should be sorted as "German" since that comes first, but found as "German" and "English").
- A German-language song:
"ger" (or "deu")
Sorted and found under "ger".
- A German song with some lines of English in it:
"ger" and "eng" - since the main part is in German (not "eng" and "ger"!)
Sorted under "ger" but found under both "ger" and "eng".
- A duet sung in English, with many Italian parts:
"eng" and "ita" - since the main part is still English (not "ita" and "eng")
Sorted under "eng" but found under both "eng" and "ita".
- An instrumental title:
Should not have a TLAN tag (or maybe an empty one). No reason to show it as "instrumental", though - it might just not have been tagged and still be a song with lyrics. (In my opinion, it just might be a good idea to set up a de-facto standard of "No TLAN=untagged" and "Empty TLAN=instrumental".)