Compare source<->dest  tools (0.9.4) / Merging Strings together

-Set Source and dest language as same (en-en or fr - fr for example)
-Open/load fallout4.esm
-Menu tool->Load .string as translation -> choose/load the strings you want to compare
-Menu tool ->Compare source<>dest, choose of you want to tag differences or matches.

Note1: by tagging matches you can merge strings together:

-load fallout4 esm with the reference default strings
-Menu tool->Load .string as translation -> choose/load the 1st set of string  you want to merge (choose replace everything)
-Menu tool ->Compare source<>dest, choose tag: matches.
-Menu tool->Load .string as translation -> choose/load the 2nd strings you want to merge   (choose replace string tagged as partial (f2)
-Done! the too strings are merged

Note2:
If you want to reset the string status between comparisons, you can use Ctrl-F1 (validate all translations)

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How to merge string esp based modifications in localized fallout4 .strings

There are a lot of mods that just change name or descriptions for items, and a lot of them actually choose to use esp.

Esp gets rid of the .strings references, since all text are embedded, this can allows compatibility with other mods that use .strings files (like new dialogs or Full Dialogue Interface), but this can also break compatibility with other mods that actually modify the same item.
Here is how you can just take the strings from modified esp, and put them in your fallout4.strings files (a long as the esp is in the same language, of course

1)Backup:
First thing to do: make a backup of your strings file
-go into you \Steam\steamapps\common\Fallout 4\Data\Strings
-make a copy of the file Fallout4_xx.STRINGS ; xx is the reference of your language 

2)Options:
-start  Fallout4translator 
-In the options (menu options ->options) set the Source language  *and* the destination language as the same: the one your are using in game 

• menu file->open fallout4.esm
• menu tool->espCompare choose the esp you want to merge, choose replace everything, click ok
• menu file->export strings