Yes. You should learn all three tiers of the ancient language. But you should only use the tablets when you do not want to raise the attribute by normal training anymore. For example your STR mercanary could raise STR to 94 or even 124 (that might actually be ambitious...). And after that use the tablets and potions to raise it even higher. But you could use the mana boni earlier because you do not want to train that anyway...

You should also learn the alchemical recipe for the permanent potion of your main attribute for the same purpose.

Yes, once skills become too expensive to train easily you'll want move onto non-training increases. These don't scale like LP costs so a +2 item always adds +2, etc. Languages are expensive but you end up gaining far more than you would spending those LPs at the higher levels.

You'll also need lockpicking, if you don't have it already, since many of the best tablets are in locked chests.

Thanks both of you it really helped.

For a total cost of 30 LP to learn them you will get:

+6% two-hander / +6% one-hander / +6% bow / +6% crossbow skill,
+12 STR / +6 DEX
+18 mana
+ 105 to life energy

...and no way you can get these cheaper than that.

Much agreed, I would probably recommend learning the first variant after you've gotten your primary skills to a reasonable level (34+) and then not using the tablets related to your primary skills until they're MUCH higher.

The reason being that increasing skills by normal means increases in costs in increments. At level 30 of a skill the cost increases by 1 and for every 30 after that it increases by another 1.

It's also recommended to increase a skill to immediately below the increment increase (29), then boosting it by 5, for efficient leveling.

You did not find a lot of tablets, did you? You can get a lot more than that...

+22 strength
+24 dexterity
+42 mana
+205 life energy
+24% one-hander
+20% two-hander
+30% bow
+20% crossbow

Those are the boni of all tablets you can find in the game...
You can get up to 28 additional mana by giving those other kind of tablets to vatras. But for that you do not need any language skills.

Actually I did find them all, and not only once.
However, you are correct. I was probably thinking of bonuses from each individual tablet (of no particular rank, for that matter).

Good thing you came up with the complete info.