In order to add you own language to a mod, it's pretty simple.

1- Locate the targeted mod in the workshop directory :

"Steam/steamapps/workshop/content/489630/MOD ID (mine is 2948573618)"

2- Open the file (mine is 2948573618) and go to Data/Core/Languages

This is the place where you can find folder for each languages already available for the mod (for mine, you'll find an "English" and "French" file).

3- Create a new folder in Data/Core/Languages for the language you want to add to the mod.

Languages available to Gladius : Relics of War :

English/French/German/Russian/SimplifiedChinese/Spanish

I don't know if a language which would be out of this list would work.
Anyway, the file you create must be one of the language available (identical to the examples above).

4- Copy and paste the content of the original folder of your choice (English or French for me) to the new one you created

For MIA (my mod) you'll have 5 folder (Features/GUI/Items/Traits/Weapons).

5- Open and translate each line (Name/Description/Flavor) in the language you want

Each file (except Traits, just ignore him) need to be translate.
You must change the "value=" line.
Don't touch the "entry name= xxxxxxx" and the " and the "/> for each line you'll translate. Otherwise it won't work.

Example for the"Items" file :

FRENCH =>

<entry name="BatteriedeVaul" value="Batterie de Vaul"/>
<entry name="BatteriedeVaulDescription" value="Augmente le taux de génération d'énergie."/>
<entry name="BatteriedeVaulFlavor" value="La batterie de Vaul est un artefact ancien capable de générer une quantité constante d'énergie. Bien que le taux de génération d'énergie ne soit pas élevé, il parait illimité. Selon les xénos-archéologues, un tel artefact pourrait être d'origine Necron ou plus ancien encore. Mais d'après les quelques écris parcourus par l'ordo xenos, l'artefact serait l'une des premières pierres-esprits façonnées par l'antique dieu des Eldars, Vaul."/>

TO ENGLISH =>

<entry name="BatteriedeVaul" value="Vaul's Battery"/>
<entry name="BatteriedeVaulDescription" value="Increases the rate of energy generation."/>
<entry name="BatteriedeVaulFlavor" value="Vaul's Battery is an ancient artifact capable of generating a constant amount of power. Although the energy generation rate is not high, it seems limitless. According to xenos-archaeologists, such an artifact could be of Necron origin or even older. But according to the few writings traversed by the ordo xenos, the artefact would be one of the first stone-spirits crafted by the ancient god of the Eldar, Vaul."/>

If you have question, don't hesitate. If you want to share your translation, just tell me.