I looked at your strings.
I don't like some.

#: ../src/about.c:139
msgid ""
"    EasyTAG is an utility for viewing and editing tags for MP3, MP2, FLAC, "
"Ogg Vorbis, MP4/AAC, MusePack and Monkey's Audio files. Its simple and nice "
"GTK+ interface makes tagging easier under GNU/Linux."
Whitespace at beginning.

#: ../src/about.c:329 ../src/easytag.c:230
msgid "E-mail: %s"
http://developer.gnome.org/gdp-style-guide/stable/gnome-glossary-generic-terms.html.en says to use "email".

#: ../src/about.c:446
msgid "ID3v2.3 tags writting support"
typo

#: ../src/bar.c:235
msgid "CD Data _Base Search…"
#: ../src/cddb.c:239
msgid "CD Data Base Search"
#: ../src/cddb.c:3001
msgid "The path for 'Local CD Data Base' was not defined"
Not sure - maybe "Database"?

#: ../src/bar.c:272
msgid "Browse _Sub-directories"
Not sure about spelling of "Sub-directories"

#: ../src/browser.c:3036 ../src/misc.c:1929 ../src/prefs.c:670
msgid "Orig. Artist"
#: ../src/easytag.c:877
msgid "Orig. Artist:"
Is there a reason for the abbrev? If the UI requires to keep the string short a translator comment would be good.

#: ../src/browser.c:3091
msgid "… file(s)"
I don't understand this string. Is that shown when the number of files is still indexed and not known?

#: ../src/browser.c:3711
msgid "Rename the directory '%s' to : "
#: ../src/cddb.c:283
msgid "Request CD database :"
#: ../src/cddb.c:373
msgid "Words :"
#: ../src/cddb.c:426
msgid "Search In :"
#: ../src/cddb.c:509
msgid "included : funk, soul, rap, pop, industrial, metal, etc."
#: ../src/cddb.c:522
msgid "Results :"
#: ../src/cddb.c:529 ../src/misc.c:1970
msgid "Search :"
#: ../src/cddb.c:710
msgid "Set Into :"
#: ../src/cddb.c:775 ../src/prefs.c:1409
"When activating this option, the Levenshtein algorithm (DLM : Damerau-"
#: ../src/easytag.c:207
msgid "Variable EASYTAGLANG defined. Setting locale : '%s'"
#: ../src/easytag.c:209
msgid "Setting locale : '%s'"
#: ../src/misc.c:1373
msgid "Write info using :"
#. Where...
#: ../src/misc.c:1987
msgid "In :"
#: ../src/prefs.c:164
msgid "Default directory :"
#: ../src/prefs.c:262
msgid "Display changed files in list using :"
#: ../src/prefs.c:287
msgid "Sort the file list by :"
#: ../src/prefs.c:386
msgid "Player to run :"
#: ../src/prefs.c:427
msgid "Max number of lines :"
#: ../src/prefs.c:474
msgid "Convert filename extension to :"
#: ../src/prefs.c:596
msgid "Write the track field with the following number of digits :"
#: ../src/prefs.c:599
msgid ""
"obtain a number with 'n' digits (Ex. with two digits : '05', '09', '10' …). "
#: ../src/prefs.c:614
msgid "Ogg Vorbis Files : Write also the comment to the XMMS format"
Whitespaces around the colon



#: ../src/browser.c:4029
msgid "Rename Directory Error"
#: ../src/browser.c:4059
msgid "Directory Rename Error"
Could those be merged?

#: ../src/cddb.c:228
msgid "Time"
Translator comment - like "right now it's 2:15PM"? Like "length" / "period"?

#: ../src/cddb.c:434
msgid "All Fields"
Assuming this and the followup values are options for "Search in" all these might be better with translator comments because of declination / cases.
For German, "Suchen in:" "Alle Felder" would be wrong, should be 
"Suchen in: Allen Feldern".

#: ../src/misc.c:1990
msgid "the File Name"
#: ../src/misc.c:1992
msgid "the Tag"
Same here. Looks like a nominative but isn't, as it's after "In:".

#: ../src/cddb.c:469
msgid "Newage"
New Age?

#: ../src/cddb.c:514
msgid " Categories "
Spaces are ugly, proper padding/margin for button text would be better

#: ../src/cddb.c:2466 ../src/cddb.c:2811
msgid "Found %d matching album(s)"
#: ../src/cddb.c:2921
msgid "%d file(s) selected!"
#: ../src/cddb.c:3301
msgid "Found %d matching album(s) for DiscID '%s'"
#: ../src/cddb.c:3896
msgid "You are applying %d lines of the CDDB results to %d lines in the list of files"
https://live.gnome.org/TranslationProject/DevGuidelines/Plurals

#: ../src/cddb.c:3002
msgid "Enter it in the preferences window before using this search."
Who is "it"?

#: ../src/easytag.c:195
msgid "Starting EasyTAG %s (PId: %d)…"
--> "Starting EasyTAG version %s (PId: %d)…"

#: ../src/easytag.c:474
msgid "Read Only File"
Verb? Adjective? 
https://live.gnome.org/TranslationProject/DevGuidelines/Use%20comments

#: ../src/easytag.c:493 ../src/easytag.c:4427
msgid "?, Layer ?"
Got milk!

#: ../src/easytag.c:3176
msgid ""
"Can't remove old directory\n"
"'%s'!\n"
"(%s)"
#: ../src/easytag.c:3192
msgid "Cannot remove old directory '%s': (%s)"
I don't get the difference between these two strings.

#: ../src/easytag.c:3212
msgid ""
"Can't move file '%s'\n"
" to \n"
"'%s'!\n"
"(%s)"
Why all those linebreaks?

#: ../src/easytag.c:4688 ../src/easytag.c:4697
msgid ""
"You have probably found a bug in EasyTAG. Please, send a bug report with a "
"gdb backtrace ('gdb easytag core' then 'bt' and 'l') and information to "
"reproduce it to easytag@gmail.com"
You don't want no bugtracker?

#: ../src/scan.c:2685
msgid "Convert '_' and '%20' to ' '"
#: ../src/prefs.c:1028 ../src/prefs.c:1053
msgid "Convert underscore character '_' and string '%20' to space ' '"
#: ../src/easytag.c:4601
msgid "Convert '_' and '%20' to spaces"
Could these be merged?

#: ../src/vcedit.c:405
msgid "Need to save extra headers - TODO!!"
Should that be marked for translation?

Also there are dozens of missing mnemonics in the menus.